Add coverage vs ANI output to pyani plot

[widdowquinn]

Summary:

Add a scatterplot of coverage vs ANI to the pyani plot output

Expected Output:

This scatterplot will enable at a glance interpretation of results in an analysis set. It also makes output visually comparable to the data in Barco et al. (2020).

We could provide a choice of several colouring/labelling options.

• most basic would be a plain scatterplot showing coverage vs ANI
• it would be useful to have an indication of class (as given in classes.txt or genus/species label (inferred from labels.txt, perhaps) indicated as colour or symbol.
• I would very much like to have a plot where the result of pyani classify is shown on the plot as symbol/colour differences - possibly specifying an ANI level, and indicating which cliques are 'active' at that level.