Make weather-model-graphs report its own version with weather_model_graphs.__version__ attribute

[leifdenby]

Currently weather-model-graphs doesn't expose its own version using the __version__ attribute that is common for python packages to do. mllam-data-prep already implements this by adding the following to mllam_data_prep/__init__.py:

import importlib.metadata

try:
    __version__ = importlib.metadata.version(__name__)
except importlib.metadata.PackageNotFoundError:
    __version__ = "unknown"