Add internal _degeneratize_R and _degeneratize_I

[hyrodium]

This PR fixes #325.

Before this PR

julia> using BasicBSpline

julia> Pd1 = BSplineSpace{2}(knotvector" 21 3")
BSplineSpace{2, Int64, KnotVector{Int64}}(KnotVector([2, 2, 3, 5, 5, 5]))

julia> Pd2 = BSplineSpace{3}(knotvector"212 4")
BSplineSpace{3, Int64, KnotVector{Int64}}(KnotVector([1, 1, 2, 3, 3, 5, 5, 5, 5]))

julia> BasicBSpline.changebasis_I(Pd1, Pd2)
ERROR: MethodError: no method matching isless(::Nothing, ::Nothing)

Closest candidates are:
  isless(::Any, ::Missing)
   @ Base missing.jl:88
  isless(::Missing, ::Any)
   @ Base missing.jl:87

Stacktrace:
 [1] <(x::Nothing, y::Nothing)
   @ Base ./operators.jl:343
 [2] <=(x::Nothing, y::Nothing)
   @ Base ./operators.jl:392
 [3] >=(x::Nothing, y::Nothing)
   @ Base ./operators.jl:416
 [4] (::Colon)(start::Nothing, stop::Nothing)
   @ Base ./range.jl:7
 [5] _find_j_range_I(P::BSplineSpace{2, Int64, KnotVector{Int64}}, P′::BSplineSpace{3, Int64, KnotVector{Int64}}, i::Int64, j_begin::Int64, j_end::Int64)
   @ BasicBSpline ~/.julia/dev/BasicBSpline/src/_ChangeBasis.jl:405
 [6] _changebasis_I(P::BSplineSpace{2, Int64, KnotVector{Int64}}, P′::BSplineSpace{3, Int64, KnotVector{Int64}})
   @ BasicBSpline ~/.julia/dev/BasicBSpline/src/_ChangeBasis.jl:527
 [7] changebasis_I(P::BSplineSpace{2, Int64, KnotVector{Int64}}, P′::BSplineSpace{3, Int64, KnotVector{Int64}})
   @ BasicBSpline ~/.julia/dev/BasicBSpline/src/_ChangeBasis.jl:350
 [8] top-level scope
   @ REPL[4]:1

After this PR

julia> using BasicBSpline

julia> Pd1 = BSplineSpace{2}(knotvector" 21 3")
BSplineSpace{2, Int64, KnotVector{Int64}}(KnotVector([2, 2, 3, 5, 5, 5]))

julia> Pd2 = BSplineSpace{3}(knotvector"212 4")
BSplineSpace{3, Int64, KnotVector{Int64}}(KnotVector([1, 1, 2, 3, 3, 5, 5, 5, 5]))

julia> BasicBSpline.changebasis_I(Pd1, Pd2)
3×5 SparseArrays.SparseMatrixCSC{Float64, Int32} with 7 stored entries:
  ⋅   0.888889  0.222222   ⋅         ⋅ 
  ⋅   0.111111  0.777778  0.666667   ⋅ 
  ⋅    ⋅         ⋅        0.333333  1.0

[codecov]

Codecov Report

Merging #342 (678a931) into main (474e85d) will decrease coverage by 0.46%.
The diff coverage is 77.14%.

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
- Coverage   97.02%   96.56%   -0.46%     
==========================================
  Files          14       14              
  Lines        1547     1574      +27     
==========================================
+ Hits         1501     1520      +19     
- Misses         46       54       +8     

Files	Coverage Δ
src/_BSplineBasis.jl	99.28% <100.00%> (ø)
src/_ChangeBasis.jl	94.08% <100.00%> (+0.10%)	⬆️
src/_BSplineSpace.jl	93.85% <60.00%> (-4.26%)	⬇️

[hyrodium]

Another case:

julia> using BasicBSpline

julia> Pd1 = BSplineSpace{3}(knotvector"44 3  2 121 ")
BSplineSpace{3, Int64, KnotVector{Int64}}(KnotVector([1, 1, 1, 1, 2, 2, 2, 2, 4, 4, 4, 7, 7, 9, 10, 10, 11]))

julia> Pd2 = BSplineSpace{4}(knotvector"55 62 3 2121")
BSplineSpace{4, Int64, KnotVector{Int64}}(KnotVector([1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 4, 4, 4, 4, 4, 4, 5, 5, 7, 7, 7, 9, 9, 10, 11, 11, 12]))

julia> changebasis_I(Pd1, Pd2)
13×22 SparseArrays.SparseMatrixCSC{Float64, Int32} with 41 stored entries:
⎡⠙⢦⡀⠀⠀⠀⠀⠀⠀⠀⠀⎤
⎢⠀⠀⠈⠳⣄⢀⣀⡀⠀⠀⠀⎥
⎢⠀⠀⠀⠀⠀⠀⠙⠿⠶⣤⡄⎥
⎣⠀⠀⠀⠀⠀⠀⠀⠀⠀⠈⠁⎦

julia> Matrix(changebasis_I(Pd1, Pd2))
13×22 Matrix{Float64}:
 1.0  0.25  0.0  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.75  0.5  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.5  0.75  0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.25  1.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  1.0  0.25  0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.75  0.5  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.5  0.75  0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.0  0.25  1.0  0.0  1.0  0.75  0.555556   0.111111   0.0       0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.25  0.388889   0.555556   0.222222  0.0  0.0  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0555556  0.316667   0.644444  0.7  0.1  0.0       0.0         0.0        0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0166667  0.133333  0.3  0.9  0.666667  0.0833333  -0.0277778  0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.333333  0.75       -0.0277778  0.0
 0.0  0.0   0.0  0.0   0.0  0.0  0.0   0.0  0.0   0.0  0.0  0.0  0.0   0.0        0.0        0.0       0.0  0.0  0.0       0.166667    1.05556    0.0

[hyrodium]

Oh, _degeneratize_R and _degeneratize_I should be _nondegeneratize_R and _nondegeneratize_I.