Adaptive softening for the gas

Implements adaptive softening for the gas when using the Wendland-C2 kernel (i.e. the kernel used in the gravity solver). Equations taken straight from https://ui.adsabs.harvard.edu/abs/2007MNRAS.374.1347P/abstract but with the terms altered for the Wendland kernel.

Changes:

• Add new config option to activate the adaptive softening (--with-adaptive-softening),
• The gravity scheme uses the gas'h as epsilon
• A min and max can be applied via the gravity section of the parameter file.
• The gravity loops and initial comms are taking place after the hydro ghost.
• The hydro scheme (SPH only) includes extra momentum terms accounting for the change in softening.
• the "baryon softening" constants given in the parameter files then only apply to stars and BHs.

Todo:

• Same for the other hydro schemes.
• Document the new behaviour in the RTD.
• Update the unit tests and tools.c

Closes #879 (closed).