Persistence diagram plotting

[lizliz]

The Draw.drawDgm function needs modernizing.

• Add input ax so that we can control things
• Add input kwargs so that things can get passed to the drawing function from outside. For example, it would be nice to be able to add a label so that when plotting lots of diagrams, the labels can be controlled.
• Make the diagonal always black or grey. Perhaps dashed. Also the bounding box doesn't actually touch which seems silly.