Add Supersymmetric Functions and Bases

SageMath currently has various examples of Hopf Algebras implemented, such as symmetric functions, quasisymmetric functions, word-quasisymmetric functions, etc. along with some of their bases. This Google Summer of Code project aims to implement an example of Hopf Algebras- the supersymmetric functions along with bases such as (double) powersum, elementary, homogeneous and Schur supersymmetric functions. The addition of supersymmetric functions and these bases would aid in research computation of their characterization and combinatorial interpretation.