(set_star) shifted all the work in setup_star into set_star routine

[danieljprice]

Type of PR:
cleanup / structural changes to set_star

Description:

The aim of this change is to be able to perform all the functionality of setup_star from any setup routine in phantom. This is with the intention of making several "two-step" setup-and-moddump procedures into a single, streamlined procedure. For example, the TDE setup could add a star, relax the star and place it into orbit without needing the moddump procedure. Similarly, this change allows implementation a binary, triple or hierarchical star setup with multiple stars read from MESA tables.

The changes to the code at the moment are purely structural, there is no change in functionality:

• added new set_star routine/module that does all the steps in setup_star
• set_star provides routines to read/write/ask for all the options needed to setup and relax a single stellar model
• teased apart the options that would be per-star from the equation of state options that are per-setup

Testing:
Testing on AGB stellar model with sink particle core, ensuring setup procedure proceeds as previously

Did you run the bots? no

Did you update relevant documentation in the docs directory? no

[themikelau]

Looks okay.