GEAR sink : Improve swallowing conditions

This MR splits !1943 (merged) into a smaller chunk.

Changelog:

• When swallowing gas, we take into account the internal energy.
• Sink swallowing also has the inner accretion radius check (if r < f_acc r_cut_off, then swallow without passing further checks). This is now more consistent concerning gas swallowing.
• The potential normalisation constant has been changed from c(a) = 0 to c(a) = - 0.5 \cdot \ddot{a} a r^2.
• Add a function in the cosmology module to compute \ddot{a}.
• Improve function docs in the two sink modules.

Additional note:

• I'm unsure about the \ddot{a} computations. Let me know if there is anything wrong.