improved mostly step merged with merger object (especially WD mergers)…

[ezapartas]

…), merger center value calculation and allowing co_core_mass in detached

• 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).
• deepcopy of the merged_star object, not just a pointer to one of the two singlestar objects
• more verbose in step_merged mainly, and step_detached
• added co_core_mass as a potential option in the matching criteria in step_detached

[tassos25]

@dimsour94 and @kasdaglie should review this PR. @ezapartas has been using it and expects no issues.