Initialising rhoN, rhoM and iDof fields to VSMALL instead of 0.0 to avoid...

Initialising rhoN, rhoM and iDof fields to VSMALL instead of 0.0 to avoid floating point errors or the need to skip field calculation.  Rewritten initialisation function to fill on a cell-by-cell basis, filling into randomly distributed positions in volume weighted selected tetrahedrons (decomposed cells) - much faster.