Derlog updates
Created by: gcarleo
This PR mostly adds a mechanism to have look-up table based computation of derivatives, partially addressing #73 (closed) . These optimized updates can be implemented explicitly by machines that want to exploit further speed-ups with caching precomputed information. By default, however new machines do not need to implement these methods, since they have a default implementation (not using LookUp tables) in AbstractMachine
.
As an experimental feature, variance minimization is added to VMC. This needs further tests, but it's good to have it in the interface at least.