Gravity multi dt

This implements a small change to the top-level multipole drifting strategy.

It should have no side effect on the hydro business but I'd rather be sure. Could you check that all your usual cases are not broken by this ? Thanks !

Added 1 commit:

• fb11828e - Only construct multipoles if the cell has gparts

Added 3 commits:

• a0931f61 - Zero the multipoles when rebuilding if there are no gparts.
• d097d467 - Force a rebuild every so often when gravity is on.
• 08b2c5fb - Code formatting

Looks good for me, SPH-wise, accepting.

Status changed to merged

mentioned in commit a380050c

mentioned in issue #287 (closed)

Thanks !