Bugfix/improve vnmult restore handling issue 63

• Make sure vnm_init is always initialised

• Change vnmult initialisation to -1

• Always initialise vnmult to the value of vnm_init

  This should make sure that restarted jobs set vnmult to the value stored in the restart file rather than defaulting to 1 as would happen otherwise.

• Add vnmult to the set of things saved/restored during a timestep change

• Make sure vnmult is restored during a timestep change before the collision operator is reinitialised.

• Correct logic

This attempts to address the issues raised in issue #63.

One possible remaining issue is that when we change vnmult through calls to get_verr during the advance phase, we end up modifying the collision operator but do not recalculate the response matrices. The size of the error introduced due to this may be something of interest to explore in the future.