Add an interface to Singular

[vsht]

The new partial fractioning library presented in https://arxiv.org/pdf/2008.13194.pdf
seems like a good motivation to add an interface to Singular CAS

In fact, such an interface for older Mma versions already exists

https://www3.risc.jku.at/research/combinat/software/Singular/

and is open source, so that one could fork it and keep only the needed functionality.