rjdirisio/pyvibdmc: MPI, NN Descriptors, and More API improvements

MPI parallelization was added for potential calls through MPI4Py. Molecular descriptors for NN-DMC were formalized in this version, and they are documented now. Users are also now allowed to add additional_timesteps to their DMC simulations, and there will always be a checkpointed file at the end of the simulation. Users may also extract the vref array as well as the walkers from the checkpointed DMC object.