Reconsider switching to the L-BFGS minimizer as standard minimizer of the multi sampler states

[andrrizzi]

The MultiStateSampler uses the FIRE minimizer by default, which has caused us some problems recently. If we can't fix it, we can either switch back to the L-BFGS minimizer, or try to minimize also with L-BFGS if the energies are still high after FIRE.