Implement and test two-layer solver

The problem

At the moment, we don't have any examples of actually solving a model.

Definition of "done"

• we implement and test solving the two-layer model

Additional context

A complete implementation is here: https://gitlab.com/magicc/fgen/-/blob/c98fa9a5cd1e9f6d66a3104e2291bbc74a47097f/tests/test-data/two_layer_model/two_layer_model_solver.f90#L46 and https://gitlab.com/magicc/fgen/-/blob/c98fa9a5cd1e9f6d66a3104e2291bbc74a47097f/tests/test-data/two_layer_model/two_layer_model_solver.yaml

Getting these tests passing will also require a solution to #22 (closed)

We can pass the tests without a solution to #20