Fallback in test if raven_hydro is not installed (#372)

Fixes #371 

TODO: review dependencies to make raven_hydro a "soft" dependency ?

tests/test_emulators.py

@@ -1,10 +1,19 @@
 import datetime as dt
+import warnings
 
 import numpy as np
 import pytest
 import xarray as xr
 from packaging.version import Version
-from raven_hydro import __raven_version__
+
+try:
+    from raven_hydro import __raven_version__
+except ImportError:
+    warnings.warn(
+        UserWarning,
+        "The `raven-hydro` library is not installed. Setting '__raven_version__' to '0'.",
+    )
+    __raven_version__ = "0"
 
 from ravenpy import Emulator
 from ravenpy.config import commands as rc