Timestep conditions: implement validation, add `teff` as an option

[TomWagg]

Two main sets of changes here:

• Validation: checkstate.py now checks whether a column is actually present in the CHECKSTATE_COLUMNS and throws an error if it isn't rather than fails silently. I also added a check that there aren't more than 15 columns if some insane users wants to condition on 16 variables because that will break it haha
• New condition variables: teff_1 and teff_2 can now be used in timestep_conditions just like the other variables 😄

I also added some instructions to checkstate.f in case anyone else needs to add more condition variables one day.

p.s. This is my first attempt at coding in fortran so please do check I didn't break something (it doesn't seem like I have but just in case).

This will fix #609

[codecov[bot]]

Codecov Report

Attention: 306 lines in your changes are missing coverage. Please review.

Comparison is base (8772c07) 86.91% compared to head (a2f556c) 87.74%. Report is 22 commits behind head on develop.

Files	Patch %	Lines
cosmic/src/assign_remnant.f	9.39%	193 Missing :warning:
cosmic/src/zfuncs.f	6.67%	84 Missing :warning:
cosmic/src/hrdiag_remnant.f	64.00%	18 Missing :warning:
cosmic/checkstate.py	25.00%	3 Missing :warning:
cosmic/src/checkstate.f	0.00%	3 Missing :warning:
cosmic/src/evolv2.f	0.00%	3 Missing :warning:
cosmic/evolve.py	0.00%	1 Missing :warning:
cosmic/utils.py	85.71%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #610 +/- ## =========================================== + Coverage 86.91% 87.74% +0.83% =========================================== Files 40 42 +2 Lines 25542 25695 +153 =========================================== + Hits 22198 22544 +346 + Misses 3344 3151 -193 ```

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