Further plot updates and fix observed contour lines

[dabail10]

Description of changes:

• Please add an explanation of what your changes do and why you'd like us to include them.

• Fix the observed contour lines in the spatial plots.
• add back in import of nc_time_axis
• Add the "maximum" ice concentration for contour plots and Labrador Sea plots

All PRs Checklist:

• Have you followed the guidelines in our Contributor's Guide?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you made sure that the pre-commit checks passed (#8 in Adding Notebooks Guide)?
• Have you successfully tested your changes locally?

New notebook PR Additional Checklist (if these do not apply, feel free to remove this section):

• Have you hidden the code cells (#8 in Adding Notebooks Guide) in your notebook?
• Have you removed any unused parameters from your cell tagged with parameters? These can cause confusing warnings that show up as DAG build with warnings.
• Have you moved any observational data that you are using to /glade/campaign/cesm/development/cross-wg/diagnostic_framework/CUPiD_obs_data and ensured that it follows this format within that directory: COMPONENT/analysis_datasets/RESOLUTION/PROCESSED_FIELD_TYPE?

[TeaganKing]

Let me know if you still want to make additional changes or if this is ready?

[mnlevy1981]

Besides the comment about the directory structure of the obs datasets, I'm a little confused about how path_nsidc is defined inside plot_diff.py. Can this be an argument passed in from the notebook, so this function works on computers that don't have access to campaign store? A couple of related questions:

1. is path_nsidc the right variable name? (Is this still NSIDC data? Isn't HadOIBl coming from UKMO?)
2. Should we have something like obs_data_root that is set to /glade/campaign/cesm/development/cross-wg/diagnostic_framework/CUPiD_obs_data/ice/analysis_datasets in config.yml and then we build up all the other path_ variables from that? path_nsidc = os.path.join(obs_data_root, 'hemispheric_data', 'NSIDC_SeaIce_extent'), path_hadoibl = os.path.join(obs_data_root, '1x1', 'HadOIBl'), etc?

This might require a little clean up in the other ice notebooks and .py files, but it will set a good precedent for later updates

[mnlevy1981]

It looks like you added the 1x1 files to /glade/campaign/cesm/development/cross-wg/diagnostic_framework/CUPiD_obs_data/ice; can we move them to /glade/campaign/cesm/development/cross-wg/diagnostic_framework/CUPiD_obs_data/ice/analysis_datasets/1x1? (The analysis_datasets/ directory exists, but the 1x1/ subdirectory does not)