Adaptive gas softening

Add the option to run with adaptive softening for the gas.

added feature request label

assigned to @matthieu

Base stuff from master thesis exists. For now limited to cubic spline kernel and no cosmo terms.

added SPH gravity labels

@rcrain and @chaikin since you both expressed interest, I have put some code together in the branch adaptive_softening.

It needs compiling with --with-adaptive-softening=yes. The gas only then uses adaptive (comoving) softening linked to the SPH kernel.

Caveats:

• does not yet work over MPI.
• A small correction term is missing but that's likely having no impact in real life applications with cooling.
• What to do at the point of star formation needs to be decided.

Usage:

• Nothing to do by default; it just runs.
• The range of softenings can be limited to a min/max using the parameters Gravity:min_adaptive_softening and Gravity:min_adaptive_softening. (Both in internal comoving units)

That's great, thanks, We'll almost certainly want to give this a test as part of the cluster particles project.

changed title from Adaptive softening to Adaptive gas softening

@yuyttenhove might be interested too for the MM scheme.

mentioned in merge request !1847 (merged)

mentioned in commit 43c583a7

closed with merge request !1847 (merged)