Render ewatercycle._forcings package on readthedocs

[sverhoeven]

Currently the ewatercycle._forcings does not show up on https://ewatercycle.readthedocs.io/en/latest/autoapi/ewatercycle/index.html#subpackages .

This makes it impossible to see the docstrings from a browser (still works from editor/notebook/repl).

It would be nice if the _forcings module could be made public.

[BSchilperoort]

I guess this is due to the autodoc skipping these modules. Changing _forcings to forcings shouldn't break anything and make that work.