Allow returning local estimators from `expect` and `expect_and_grad`

Created by: femtobit

This PR introduces an (experimental) option _return_estimators for MCState.expect and MCState.expect_and_grad, which optionally returns the local estimators O_loc(s) = <s|O|ψ>/<s|ψ> and not just their statistics.

This is necessary for implementing #1140 and potentially other VMC regularization methods or preconditioners and also can be useful for debugging purposes (e.g., to check MCMC chain properties beyond those contained in statistics).