Use a single config type for both old and new diagnostics

Replaces the separate config types and machinery in src/gs2_diagnostics.fpp and src/diagnostics/diagnostics_config.f90 with a single implementation in src/diagnostics_base_config.f90. There are also two subroutines, apply_old_defaults and apply_new_defaults that change values in the input config variable to either the old or the new defaults. The actual implementation has the default values corresponding to the old module, but only because that’s the version I copied – at some point we will need to choose which defaults to keep.

The tests pass on my machine, and checked the outputs using #410. This is sort of a starting point for discussing where to go next.

One possible next step is to make sure all the inputs are functional for both modules, trying to reuse code where possible.