Update Chebyshev.jl

[putianyi889]

Added virtual fields b and a so that it can be considered as a Jacobi space.

[dlfivefifty]

Can you add tests?

[codecov]

Codecov Report

Merging #4 into master will increase coverage by 1.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   54.83%   55.85%   +1.01%     
==========================================
  Files          26       26              
  Lines        2057     2025      -32     
==========================================
+ Hits         1128     1131       +3     
+ Misses        929      894      -35

Impacted Files	Coverage Δ
src/Spaces/Chebyshev/Chebyshev.jl	52.63% <100%> (+1.28%)	⬆️
src/Spaces/Ultraspherical/ContinuousSpace.jl	50.39% <0%> (+0.39%)	⬆️
src/roots.jl	53.68% <0%> (+0.55%)	⬆️
...c/Spaces/Ultraspherical/UltrasphericalOperators.jl	72.22% <0%> (+1.47%)	⬆️
src/Spaces/Jacobi/JacobiOperators.jl	51.36% <0%> (+1.55%)	⬆️
src/specialfunctions.jl	69.44% <0%> (+1.87%)	⬆️
src/Spaces/Laguerre/Laguerre.jl	60.16% <0%> (+1.97%)	⬆️
src/fastops.jl	100% <0%> (+2.32%)	⬆️
src/Spaces/Hermite/Hermite.jl	42.42% <0%> (+2.42%)	⬆️
src/Spaces/PolynomialSpace.jl	83.33% <0%> (+3.02%)	⬆️
... and 1 more

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