Two-stage common envelope formalism from Hirai & Mandel 2022

[TristanBruel]

Ciao!
Here is a proposed implementation in COSMIC of the two-stage common envelope formalism presented in Hirai & Mandel 2022.
It basically relies on the idea that only the convective part of the envelope should be involved in the common envelope phase, and, if present, the radiative intershell more tightly bound to the core should only be lost through a stable mass transfer.

• When using SSE, it is based on fits from Picker, Hirai & Mandel 2024 to compute the mass of the convective envelope and its binding energy.
• When using METISSE, the mass of the convective envelope is directly interpolated from the stellar tracks via mass_conv_envelope
  It is worth noting that COSMIC technically already computes the mass of the convective envelope via the MRENV subroutine, but I could not find where these fits come from (I believe it is not from Hurley et al 2000).
  Cheers,
  Tristan Bruel