Fortran Tests

[jatkinson1000]

As part of the coupling process and developing the interface it is useful to have a testing framework.

This ensures code continues to behave as expected during refactoring, and also helps check the validity of new code being written.

This is discussed in #45 but will cover more than just regridding.

Tests:

• relu function
• Reading in NN parameters from netCDF
• Running the net with ones compared to Pytorch
• Running parameterisation with ones compared to previous results.

[jatkinson1000]

I think this is now ready to be merged back into the main repo, performing tests on the original extracted parameterisation.

My plan is to then merge that into #44 and add additional tests for the developments that have occurred there, as well as checking that the code continues to function as expected.

[dorchard]

Could we add a brief line in the README that says how to run the tests?
Otherwise looks good (nice test_utils...!)