Refactor plotting helper functions

[sebastian-schindler]

Especially plot_highdim needs refactoring:

• Structure of several completely different types of plots in one function is a bad idea, even if it allows some code sharing. Hard to see at a glance which code belongs to which plot style.
• Every added functionality that needs passing parameters to the loop or finish function needs changes at several points.
• Code sharing can still be done by putting common things in their extra functions. These can be made "private" when using a class with static methods instead of one mega function directly in the module.
• Class still allows structuring of all high-dimensional plotting styles under one umbrella (a class with static methods instead of a single function).
• Class instances allow possibility for re-using pre-computed information, and may offer other benefits in the future.
• Call signature changes from plot_highdim(data, style='corner') to no more than e.g. HighdimPlots.corner(data).

[sebastian-schindler]

After refactoring, implement new feature:

• [ ] Allow UMAP embedding to plot each cluster in its own plot to better discern the colored points.