Modules missing stubs or py.typed markers

[jarq6c]

I received this error when running mypy on a project that depends on hydrotools.nwis_client.

error: Skipping analyzing "hydrotools.nwis_client.iv": module is installed, but missing library stubs or py.typed marker

I discovered that none of the hydryotools subpackages are PEP-561 compliant, however they do use type hints. I think we should considering adding py.typed files to help users that utilize type checkers.