Allow users to override installing the raven-hydro dependency

[Zeitsperre]

The way that the requirements work right now, we absolutely need raven-hydro to be installed (either from PyPI/pip or via conda). Perhaps there could be a low-effort and safe approach to override this requirement for some special cases?

[Zeitsperre]

@julemai The issue is this line right here:

RavenPy/tests/test_emulators.py

Line 7 in dbe5fbe

from raven_hydro import __raven_version__

We need the package installed to get the version string. I can maybe propose a fix this morning, but I don't have time to release something just yet.

[julemai]

Ahhhh. That makes sense. No worries regarding releasing anything right now. I'm happy to test it in case you do at one point.

[huard]

I'd be ok to just set the version string to "-" when the user provides her own executable.

[huard]

The offending line is in the tests, so I suspect that the code "should" work even if one test breaks.