Draft: Resolve "Deduplicate model fields"

Closes #120

Adds a number of semantically descriptive type aliases in Sigmet type hierarchy as well as config. Moves fields upwards in the type hierarchy to reduce field duplication in sub-types of SigmetBase.

To test: there isn't much to test, as this is purely a refactoring — automatic tests should run as they did. Test case tests/test_sigmet.py::test_sequence was modified, as it was using an unnecessarily skeletal message instance. According to rather superificial trials with the front-end the backend works as it did.