Flux handling in Fortran

Blocked by:

• #25 (closed)
• #36 (closed)

The problem

To solve #22 (closed), we need to go through the whole stack. This is an interim issue, which just focusses on doing this in Fortran and testing in Fortran (via make test-libfgen). Basically, the idea is to just do a test which only really runs the body of this subroutine https://gitlab.com/magicc/fgen/-/blob/solver/tests/test-data/two_layer_model/two_layer_model_solver.f90?ref_type=heads#L97

Definition of "done"

• We have a test that implements the body of https://gitlab.com/magicc/fgen/-/blob/solver/tests/test-data/two_layer_model/two_layer_model_solver.f90?ref_type=heads#L97, the test should check that:
  • the model has run correctly
  • the fluxes and cumulative fluxes are consistent i.e. they are calculated as we expect

Additional context