Fully type-annotate package

[jwodder]

Running mypy on code that uses the latest version of nwbinspector fails with the error message "module is installed, but missing library stubs or py.typed marker". A quick look at nwbinspector's source indicates that it is making some use of type annotations, but a number of annotations are missing.

Please do the following:

• Fully type-annotate all public items
• Add a CI job to run mypy on the package, and ensure it passes
• Add an empty file named py.typed inside src/nwbinspector/
• Ensure that the src/nwbinspector/py.typed file is included in sdists & wheels built from the project. This may involve configuration specific to your build backend (setuptools) that is beyond the scope of this comment.
  • You can list the contents of an sdist (the dist/nwbinspector-{version}.tar.gz file created when building the project) by running tar ztf dist/nwbinspector-{version}.tar.gz on it.
  • You can list the contents of a wheel (a dist/nwbinspector-{version}-*-*-*.whl file created when building the project) by running zipinfo -1 dist/nwbinspector-{version}-*-*-*.whl on it.