Feat request: plotting functions

[hkershaw-brown]

Demo of diagnostics plots

• rank histogram
• plot evolution
  plots the temporal evolution of forecast, analysis {rmse, bias, ?} for an observation type on given levels (vertical bins)
• plot profile
  plots the spatial and temporal average of {rsme, bias, ?} on given levels (vertical bins)

[hkershaw-brown]

Note @hkershaw-brown
@icastorm method of having the total spread calculated when you are interested in it (plotting) I think is much better than having things (rmse, bias) added columns on read of the obs_seq.

[hkershaw-brown]

Matplotlib test their plots against baseline plots:
https://matplotlib.org/devdocs/devel/testing.html#writing-an-image-comparison-test