Make installation of the raven_hydro library optional

[Zeitsperre]

Pull Request Checklist:

• This PR addresses an already opened issue (for bug fixes / features)
  • This PR fixes Make raven-hydro dependency optional #384
• (If applicable) Documentation has been added / updated (for bug fixes / features).
• (If applicable) Tests have been added.
• CHANGELOG.rst has been updated (with summary of main changes).
  • Link to issue (:issue:number) and pull request (:pull:number) has been added.

What kind of change does this PR introduce?

• Moves the raven-hydro and RHF-handling code to the top-level imports.
• Adds a new ravenpy[raven-hydro] pip recipe.
• Adds a test that temporarily renders the raven_hydro and raven binary invisible using monkeypatch to test import behaviour.

Does this PR introduce a breaking change?

Yes. raven-hydro now needs to be explicitly installed. Users should be told to use the pip recipe, since the raven-hydro package hosted on conda-forge needs to be installed with an explicitly-defined version (e.g. ==0.4.0).

Other information:

I haven't yet tested this on Windows. Path patching might be exactly the same.

[Zeitsperre]

FYI @RondeauG and @sebastienlanglois

[coveralls]

coverage: 82.083% (+0.05%) from 82.037%
when pulling 8c40e55 on optional-raven-hydro
into 598795f on master.

[RondeauG]

What does it change, in concrete terms? Do we need to add raven-hydro to our YAML/pyproject files?

Edit: Since I think that we want its features in xHydro?

[Zeitsperre]

If you install RavenPy from PyPI, it will no longer come with the requirement of the source package raven-hydro (i.e. users will need to install that themselves or pass $ pip install ravenpy[raven-hydro] to get it).

If the raven-hydro Python/conda package isn't found, you need to set the PATH to the raven binary/executable; If it doesn't find either, the package will fail with a RuntimeError. Since the raven version string is provided via the Python package, the default version in this case will read 0.0.0.

I want to keep raven-hydro in the conda package, so if you use conda entirely, there will be zero changes. For a pure Python installation, compiling raven-hydro was already required, but this makes it so that you need to do that explicitly.

[huard]

Nice improvement.