VPIC crashes if the decomposition leaves the grid size as nx/ny/nz=1

[rfbird]

This is obviously not a very likely scenario, as we run on meshes much bigger than this, but it may be a useful case for testing computational intensive modules (and writing unit tests etc).

The crux of the problem seems to come down to a check that, for each dimension, does:

const float px = (nx>1) ? g->rdx : 0;

If n_=1 it will set p_ to be 0. This is then used in a division (alphadt = 0.3888889/( px*px + py*py + pz*pz ); ) so inf/nans propagates throughout anything that touches the fields.

[rfbird]

This is because this calculation uses local grid size (nx) instead of global. Should be an easy fix is the proc has access to global nx size