Fix sea-ice historical median features

[xylar]

The name of the feature needs to be exactly the same as the folder it's in (except ' ' --> '_') and the name of the component needs to be seaice instead of sea ice.

[xylar]

@alexolinhager, for future development, you may save debugging time by following these instructions:
https://mpas-dev.github.io/geometric_features/main/adding_features.html

They involve creating features and then splitting them out with gf.split(fc), which will automatically name the folders based on the feature names, components, etc. (and will catch some incorrect properties along the way).

Likewise, they use write_feature_names_and_tags(gf.cacheLocation) rather than manually editing geometric_features/features_and_tags.json to ensure that the contents of the latter file are consistent with the contents of the geometric_data files.

[xylar]

Approved by @alexolinhager in #220