Resolve "Add shortcut logic for discrete and labelled `ColorManager` setup"

This MR adds a shortcut syntax for specifying labels and corresponding colors and labels from a single mapping:

cmap:
  foo: red
  bar: blue
  baz: green

It also reworks the internals of the ColorManager to make it more easily expandable and fixes a few visual bugs.

One new feature is the addition of the discretized flag, allowing to specify whether working with discretized data; if so, some tweaks are possible during inferring of label positions and BoundaryNorm boundaries.

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
  • Parameterised tests
  • Now generating output
  • Coverage maximised
• Documentation extended or updated
  • Docstrings
  • Expand ColorManager page with example plots
  • Show examples of new syntax and resulting plots

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped

Related issues

Closes #328 (closed)