Update contour-plots to new syntax introduced in mpl v3.8

[raphaelquast]

This needs to be addressed to ensure (future) compatibility with matplotlib >= 3.10

  /home/runner/work/EOmaps/EOmaps/eomaps/shapes.py:51: MatplotlibDeprecationWarning: The collections attribute was deprecated in Matplotlib 3.8 and will be removed two minor releases later.
    self.collections = self.contour_set.collections

• ContourSet is now a Collection by itself (see: https://discourse.matplotlib.org/t/collections-attribute-deprecation-in-version-3-8/24164/5)

  • the _CollectionAccessor class for plotting contours should no longer be needed.

• [ ] TODO: update syntax to remove _CollectionAccessor.