Adding cholesky decomposition as possible choice for exact solver

Created by: fabienalet

In the Stochastic Reconfiguration part of the variational Monte Carlo, this PR adds the possibility to use Cholesky decomposition in lieu of QR by adding UseCholesky = True in the input parameters. In my tests, this considerably speeds up computations when using the exact solver. Currently UseCholesky = False by default, one could set it to True after more tests if you agree.

Ideally, one would like to speed up the iterative solver part too, but I did not manage to do so yet.