Add getindex for DunklXuDisk and tests#205

dlfivefifty wants to merge 4 commits intomasterJuliaApproximation/MultivariateOrthogonalPolynomials.jl:masterfrom

dl/rectdiskderivationJuliaApproximation/MultivariateOrthogonalPolynomials.jl:dl/rectdiskderivationCopy head branch name to clipboard