Neural Density Matrices

Created by: PhilipVinc

As discussed with @gcarleo, this PR adds support for Neural Network representations of density matrices.

An abstract base class AbstractDensityMatrix is added which stores a doubled Hilbert space and the original physical space. A first implementation of the real-valued Neural Density Matrix ansatz originally proposed on arXiv:1801.09684 is also included.

Ideally this is a first step into supporting the variational search for non-equilibrium steady states.