Allow const access to base_hmc::_z

[kedartal]

PR

Implementation in #2946

Summary:

Allow access to stan::mcmc::base_hmc::_z in constant contexts.

Description:

When integrating Stan as a library, it is many times useful to work with a const reference to a sampler, but then access to _z is blocked. The PR in #2946 suggests adding to stan::mcmc::base_hmc

const typename Hamiltonian<Model, BaseRNG>::PointType& z() const noexcept {
   return z_;
}

Current Version:

v2.24.0