Constructing from b=-1

[DanielVandH]

It's currently not possible to construct a SemiclassicalJacobi from b=-1 except to b=1,

julia> P = SemiclassicalJacobi(2.0, -1/2, -1.0, -1/2);

julia> Q = SemiclassicalJacobi(2.0, -1/2, 0.0, -1/2, P);
ERROR: Polynomials must be orthonormal
Stacktrace:
 [1] error(s::String)
   @ Base .\error.jl:35
 [2] cholesky_jacobimatrix(W::LazyBandedMatrices.Tridiagonal{…}, Q::SemiclassicalJacobi{…})
   @ ClassicalOrthogonalPolynomials C:\Users\danjv\.julia\packages\ClassicalOrthogonalPolynomials\3YYrU\src\choleskyQR.jl:53
 [3] semiclassical_jacobimatrix(Q::SemiclassicalJacobi{Float64}, a::Float64, b::Float64, c::Float64)
   @ SemiclassicalOrthogonalPolynomials C:\Users\danjv\.julia\packages\SemiclassicalOrthogonalPolynomials\GAsGB\src\SemiclassicalOrthogonalPolynomials.jl:194
 [4] SemiclassicalJacobi(t::Float64, a::Float64, b::Float64, c::Float64, P::SemiclassicalJacobi{Float64})
   @ SemiclassicalOrthogonalPolynomials C:\Users\danjv\.julia\packages\SemiclassicalOrthogonalPolynomials\GAsGB\src\SemiclassicalOrthogonalPolynomials.jl:127
 [5] top-level scope
   @ REPL[30]:1
Some type information was truncated. Use `show(err)` to see complete types.

The call for Q should probably do something like

julia> Q = SemiclassicalJacobi(2.0, -1/2, 0.0, -1/2, SemiclassicalJacobi(P.t, P.a, one(P.b), P.c, P))
SemiclassicalJacobi with weight x^-0.5 * (1-x)^0.0 * (2.0-x)^-0.5 on 0..1

The SemiclassicalJacobi(P.t, P.a, one(P.b), P.c, P) call is efficient

SemiclassicalOrthogonalPolynomials.jl/src/SemiclassicalOrthogonalPolynomials.jl

Lines 174 to 176 in a1e4c3e

	elseif iszero(Δa) && iszero(Δc) && Δb == 2 && b == 1
	# When going from P[t, a, -1, c] to P[t, a, 1, c], you can just take
	return SymTridiagonal(Q.X.d[2:end], Q.X.du[2:end])