Update top level config

[brynpickering]

Fixes issue #452

Possible TODO: allow init kwargs to be passed to the built-in example models on their initialisation (e.g., calliope.examples.national_scale(override_dict={"config.init.subset_time": ...}) -> calliope.examples.national_scale(subset_time=...))

TODO: add top-level parameter tests.

Summary of changes in this pull request:

• run and model config split into init/build/solve config keys with a flat dict layout in each.
• parameters moved out of config into a top-level parameters key with a syntax discussed in Separate top-level config and parameters  #452 and in a number of discussions.

Reviewer checklist:

• Test(s) added to cover contribution
• Documentation updated
• Changelog updated
• Coverage maintained or improved

[brynpickering]

Closing in favour of #497