Supoprt sparse matrices to represent Hpolytopes

[TolisChal]

volesti's C++ backend now supports sparse representation of Hpolytopes.
Rvolesti needs to modify its R/Rcpp functions to support also this option.

[vfisikop]

We should decide whether or not we should reuse the R class for sparse polytopes that is already in the repository
https://github.com/GeomScale/Rvolesti/blob/develop/R/HpolytopeSparseClass.R