Ground state calculation using imaginary time propagation

Created by: femtobit

This PR adds a work-in-progress implementation of ground state calculations using imaginary time propagation with the Runge-Kutta time-stepping already implemented in the feature/time_evolution branch.

(It could probably use some more tests and documentation before merging to master, but I think this is a good time for some initial feedback.)

I have added a tutorial example for the 1d Ising model, which gives the same result for the ground state energy as the RBM version. (Currently the example uses N=12 as larger N were slow. The runtime was dominated however by the construction of the sparse matrix, so that should be solved by PR #54.)