Cleanup and fixes to fdsnws importer

See merge request pevans/shakyground2!1

- Seems like behaviour of ET changed around Python 3.6-3.9.
- Add test that dictionary has meaningful content.

- Also, silence mypy.

- Allow operating on a local QuakeML event file.
- Add unit tests for this.

Prepares a dictionary with the interesting things found
by reading the QuakeML file served for a given event.

- If no focal mechanism is present, return None in
  those components of the dictionary; YAML serialises
  these as 'null'.
- Parse FM XML object for all six nodal plane angles.
- Catch float conversion problems by omitting output of these.
- Check event id looks okay.
- If called from command line, report outfile name.