Write bespoke GACODE reader to avoid pygacode import

[bpatel2107]

There were some issues with the import of pygacode due to the requirement of a fortran compiler. That was only being used to read in input.gacode files but this broke pyro on machine without fortran compilers. I've written a direct reader for that file type so we can remove the dependency on pygacode

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.58156% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 84.72%. Comparing base (734a665) to head (36c2323).

Files	Patch %	Lines
src/pyrokinetics/equilibrium/gacode.py	98.61%	1 Missing :warning:
src/pyrokinetics/kinetics/gacode.py	98.55%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## unstable #307 +/- ## ============================================ + Coverage 84.56% 84.72% +0.16% ============================================ Files 46 46 Lines 6898 6987 +89 ============================================ + Hits 5833 5920 +87 - Misses 1065 1067 +2 ```

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