fully type everything and add py.typed marker

[mikapfl]

Is your feature request related to a problem? Please describe.

If you import primap2 in a file and check the file with mypy, you get an error:

app.py:1: error: Skipping analyzing "primap2": module is installed, but missing library stubs or py.typed marker  [import]

That's of course stupid because primap2 is in fact typed, we just haven't added the correct marker to tell the world about it.

Describe the solution you'd like

Add the py.typed marker file.

Additional context

I'm unfortunately not sure what will happen with the xarray accessor and if typing works on it. May have to investigate.

[mikapfl]

Actually, we are not mypy-clean, so we need to put the work in to fix all remaining mypy issues before we can add the py.typed marker.