Support custom ordering in SubBasis

[blegat]

We should get the ordering like done in #82 for

StarAlgebras.jl/src/bases_fixed.jl

Line 105 in dd6c877

return new{T,keytype(keys),K,typeof(parent_basis),typeof(keys)}(parent_basis, keys, issorted(keys))

StarAlgebras.jl/src/bases_fixed.jl

Line 125 in dd6c877

i = searchsortedfirst(b.keys, key)