conversion of smatrix{spoly{T}} to AbstractAlgebra.Generic.Mat{spoly{T}}

[heiderich]

I would consider a conversion function such as in the title of this issue for meaningful types T (such as n_Z or n_Q) helpful.

[wbhart]

We need very many conversion functions. They will all need to be implemented for Oscar.jl. But first I'm trying to get Singular.jl to work at all and to get the basic interface in line with AbstractAlgebra.