updates after review #700 Add KGE Score

[durgals]

Please work through the following checklists. Delete anything that isn't relevant.

Development for new xarray-based metrics

• Works with n-dimensional data and includes reduce_dims, preserve_dims, and weights args.
• Typehints added
• Docstrings complete and followed Napoleon (google) style
• Reference to paper/webpage is in docstring
• Add error handling
• Imported into the API

Testing of new xarray-based metrics

• 100% unit test coverage
• Test that metric is compatible with dask.
• Test that metrics work with inputs that contain NaNs
• Test that broadcasting with xarray works
• Test both reduce and preserve dims arguments work
• Test that errors are raised as expected
• Test that it works with both xr.Dataarrays and xr.Datasets

Tutorial notebook

• Short introduction to why you would use that metric and what it tells you
• A link to a reference
• A "things to try next" section at the end
• Add notebook to Tutorial_Gallery.ipynb
• Optional - a detailed discussion of how the metric works at the end of the notebook

Documentation

• Add the score to the API documentation
• Add the score to the included list of metrics and tools