todo list
todo
decouple figures and axes

- plot functions only return figure

- cli spec for plotting vertical transects and time slices.

- cli no_plot option

- various rearrangement and renaming

- use plt.subplots everywhere

- general plotting function for simple plots with a single ax.

- remove redundant 'save' argument

- plot mean velocity components

- single specification of stats functions (nanmean etc.)