Merge VMC estimation code in _vmc and variational submodules

Created by: femtobit

This PR removes some code duplication by using the former variational.estimate_expectations code also in the Vmc driver (at least in the branch of the code that uses full der_logs and SR; for the version that relies on the vjp, this makes less sense).

netket.variational now contains only legacy code for backwards compatibility and could be removed for 3.0.

Furthermore, estimate_expectations was the last place in the code where covariance_sv was used, which is now removed. Thus, the C++ statistics are removed as well, addressing #366 (closed).