When running with VELOCIraptor, collect the DM particle in group for i/o alongside the regular gpart data.

Pass the correct units to VR. Pass more cosmology info to VR. Only update the particle fields if running VR at a snapshot dump time.

Pass the correct units to VR. Pass more cosmology info to VR. Only update the particle fields if running VR at a snapshot dump time.

Delayed foreign allocation

Closes #456

See merge request !716

Better link size estimates

Closes #521

See merge request !712

Do not count the particles in the leaves when looking for the number of foreign particles to allocate.