Implement feedback loop

See merge request !683

Add small cosmological volume example of using VELOCIraptor

See merge request !682

Add example of isolated galaxy and change hernquist potential configuration

See merge request !681

If the attribute NumFilesPerSnapshot is present in a snapshot header and has a value different from 1, return an error message.

If the attribute NumFilesPerSnapshot is present in a snapshot header and has a value different from 1, return an error message.

Fix failure to get initial partition with EAGLE_100 volume.

See merge request !672

Two fixes to the gravity code

Closes #492

See merge request !680

In the long-range gravity task, use the top-level cell for the mesh distance check, not the cell where the task is called.

When linking the hierarchical gravity tasks, do not link to the parents if we are at the super-level.

Conflicts:
	examples/IsolatedGalaxy_dmparticles/angularmomentum.py
	examples/IsolatedGalaxy_potential/angularmomentum.py
	examples/IsolatedGalaxy_potential/isolated_galaxy.yml

Correct the debugging code in the snapshot and snap dumps following the merge of the smarter drift branch.