GEAR sink improvements

This branch hosts the changes from the last few months. They are now ready to be reviewed and merged.

Todo: [x] Update documentation

Changelog

General:

• Clean the GEAR implementation details that were outside the sink module
• Clean the Default sink
• Added convenient functions to deal with sink properties changes (sink_update_sink_properties_X_star_formation(), where X = before, during, after) and hide implementation details
• Improved sink function descriptions (both in GEAR and default module)
• Updated sink function descriptions (same comment)

GEAR sink:

• Added a new potential normalization
• Added thermal term in gas accretion
• Spawn stars in a homogeneous sphere around the sink
• Give a velocity to the spawned stars
• Use the star_feedback_type instead of a dedicated sink enum.

GEAR feedback (related to sink module):

• Allow continuous IMF star particles to generate SNIa and SNII
• Cleaning and improvement in feedback.c with the will_do_feedback to regroup all cases inside this function and reduce code duplication
• Bug fix: preventing massive individual stars from exploding
• Bug fix: update the gpart's mass and velocity after feedback event (for gas and star particles)

Note: The feedback changes are not the two new mechanical feedback implementations!