Simple way for ploting CML paths with a colormap

[cchwala]

Is your feature request related to a problem? Please describe.
It is a common use case to plot CML paths on a map. Often we also want to color the CML paths based on some variable, e.g. the estimated path-averaged rainfall rate. This should be made easy with a one-line function call and be fast when plotting hunderes of CMLs. Since matplotlib is still the standard plotting tool it should be based on matplotlib.

Describe the solution you'd like
The solution here in the prototype of poligrain (still with the old name ragali) is good. It should just start with the most simple use case first, i.e. just plotting the lines with black color. Adding coloring based on a cmap should then also be added.

Describe alternatives you've considered
Using bokeh would also be possible, but it is just not that common. It could be added alter as a second backend, but matplotlib has priority.

Additional context
-none-

remaining TODOs:

• add complete docstring to first simple plot function
• rename module and function name (maybe vis.py and plot_paths()) --> trackedd in define names and structure of module files #9
• add options to color lines with cmap if a xr.DataArray is passed, but only if there is 0-length time dimension
• allow to skip through time with a slider widget in CML path plotting function #25
• Extend CML path ploting function for SMLs which have different location metadata #26

[UPDATE]: Parts of these points are done in #24. The rest will be put into new issues.

[cchwala]

Since #2 had to be merged quickly there are some things that still need to be implemented. Hence, I reopen this issue.

[UPDATE]: Moved TODO list to issue description to easily create new issues from task list items.