Merge branch 'ic-checks' into 'master'

Add checks for particles that are at same position and updates to the h_max of top-level cells.

Investigating #324 has turned up a couple of issues that we probably should
handle better. The first is particles with the same position. These are not 
allowed. The second is a mismatch between the cells h_max values and those
of the particles when running the ghost tasks for the first time and when
the initial estimates of h are small, so we get too many top-level cells 
initially and a regrid is needed (if this isn't done then the particles
test to be moved outside their cell and we need to rebuild, even though
a rebuild has just been done).

This fixes both by reporting an error when particles are found in the
same location, and updates the top-level cells h_max to those of the
particles (note this must be followed by a regrid as any sub-cells
will have the wrong h_max).

Also now reports if gparts are at the same location. That is inefficient
not a show stopper.

See merge request !370