Update `evolv2` to F90

[d-maclean]

Hi all,

As discussed earlier (and originally proposed by @xevra if memory serves), it would be a worthwhile effort to update evolv2 to use modern Fortran. This is a nontrivial effort due to the size & complexity of the file, but the potential to save developer time in debugging / crashes could be huge. I'd like to coordinate this with @xevra and other people who are interested.

I am currently trying this on my fork and will push some commits soon. Ideally I'd like to select a bunch of binaries and test by being able to reproduce them with the rewritten subroutine. This would not necessarily require rewriting the other (hrdiag/comenv/etc.) subroutines, but they will have to be updated to get state from the new module rather than a common block.

This would ameliorate the following issues:

• The odds of getting stuck in GOTO loops will be reduced (but a few might remain in other subroutines)
• common blocks will be obsoleted and replaced with the module system
• code will become much more readable, with longer line lengths
• potential for better logging capability

Other potential advantages that come to mind:

• Using constants to denote flags for better readability (e.g. kickflag == disberg_25 instead of kickflag == 5)
• standardized variable names (e.g. clear core mass names massc_he / massc_co )
• we can probably make the file shorter!