Support intersection between an Hpolytope and an ellipsoid

[TolisChal]

This PR: i) Provides a new C++ class that represents the intersection between an Hpolytope and an ellipsoid, ii) Implements C++ examples on how to sample from the intersection between an Hpolytope and an ellipsoid, iii) Exposes to R the new class using a new Rcpp module, iv) Implements R example on how to sample from the intersection between an Hpolytope and an ellipsoid, v) Provides the Dirichlet's distribution oracles in C++, vi) Provides C++ examples to sample from the Dirichlet distribution, vii) Implements R examples on how to sample from the Dirichlet distribution.