Make all the EAGLE examples run with gravity, cosmology and stars switched on. Make them dump more snapshots.

Set the compression level to 1 for the basic examples. 4 does not improve things and is just slower.

This reverts commit ccf5f553.

Improve the way we define the time of the first snapshot and statistics in cosmology and non-cosmology modes.

Changed VELOCIraptor interface to pass output file name at invoke time instead of at initialisation.

Fixed the gravity bugs reported in #363. Need to find a good rebuild criterion for gravity-only runs.

Fix to #363. Drifting decision needs to use the same multipole information as the actual tree walk. Also fix some gravity_cache padding values.

Added the EAGLE_6 test case. This is a very small cosmological volume not representative of the workload of large runs.