MultivarDecomp.modeplot() arguments - new default + fix when archiveType is not provided

[khider]

When the archiveType is not provided in the GeoSeries, the map doesn't make much sense using the defaults:

Problem 1:

The work around is to set marker='o' but I would expect that by default, I get something better than question marks when archiveType (default for marker setting) is None. I would write a condition that if archiveType is None for all GeoSeries in the MultipleGeoSeries object, then set marker to 'o'.

Problem 2:

The legend on the right doesn't make sense when there are no archiveType provided. But when using the command: scatter_kwargs={'legend':False}, the colorbar disappears.

[CommonClimate]

Solution agreed upon by the High Council on 02/09/2024:

• [x] make marker=None the default for MultivarDecomp.modeplot()
• [x] update docstring to mention the possibility of archiveType
• [x] update the docstring example of MultipleGeoSeries.pca() to specify marker = 'archiveType'

[khider]

Fixed with #525