Add sparse printing methods for sparse symplectic matrix types

[apkille]

So far, the SymplecticGivens type has been implemented, and SymplecticHouseholder as well as SymplecticButterfly types are about to be public. It would be good to have nice Base.show methods for supported types in this package, similar to the SparseMatrixCSC types in SparseArrays.jl.