Fix mypy errors around numpy functions not being strictly type hinted

[TomNicholas]

Hoping this fixes some of the typing errors @norlandrhagen flagged in #249

[TomNicholas]

Hoping this fixes some of the typing errors

This does seem to work, but would be easier to see if mypy ran in a separate CI job (#253)

No idea what's going on with the hdf5 dependency in the docs build

[norlandrhagen]

No idea what's going on with the hdf5 dependency in the docs build

For building the docs, do we need all the deps in the -e "..[test]" line?

[TomNicholas]

For building the docs, do we need all the deps in the - -e "..[test]" line?

The docs don't actually dynamically run any examples (see #83), so almost certainly we don't.

[TomNicholas]

This passes the new separate test so should be good to merge?