Option to vertically centre nodes in IPPM [MVP]

[caiw]

Fixes #363

• Can now specify y_ordinate_method="centered" as an argument to IPPMBuilder().
  • "progressive" reverts to old behaviour and is the default.
• If "centered" is specified, also supply a list-of-lists describing the serial sequence of parallel steps.
  • This could be determined automatically, but that step would be made easier by Refine IPPM graph datastructure #362, so for now it's manual.
• If "centered" is specified,f unctions within each serial step are vertically centred among each other.
• Optionally also specify avoid_collinearity=True to avoid cases where CV would induce dots or lines to overlap making graph structure impossible to visually read (as in the two examples in the demo notebook).

Also does

• Automatically scales arrowheads depending on the size of the graph.

This is a minimum-viable product

• A lot of this will be improved by Implement "extended_proj" in max filter #337, which will determine the order automatically rather than requiring specificaiton
• The public interfaces will be improved and simplified as part of Refine IPPM graph datastructure #362

Example before/after

[neukym]

Looks great...! Approved.