Corrected/enhanced merger process

[ezapartas]

Implementing and solving issues already discussed in a previous PR version #311 which is now deprecated.

• No deepcopy during merger (which was working, but led to memory and speed issues)
• However, correct caclulations of the mass weighted averages of abundances BEFORE mass changes.
• calculating center_gamma and avg_c_in_c_core for mergers whenever possible (as the core value if the cores do not merge and np.nap and mass-weighted average respectively, if the cores merge).
• added co_core_mass as a potential option in the matching criteria at track_match (mostly for mergers)

Reminder : The resulting merger product is continuing as a singlestar instance at the place of the engulfing star (i.e. if oMerging1, then star_1 would be the merged product) UNLESS a NS/BH is involved, where the latter is the only one surviving (potential Thorne-Zytkov object).