Revamp plotting methods

[MichaelClerx]

The current ones were written very much from a run from the console point of view. While that's likely the case for serious optimisation (running for hours), there'll be a lot of playing around e.g. in jupyter notebook that we should facilitate.

It could be something quite simple, e.g.

• replace headless argument in plot() method with a filename one. If you specify a filename it runs in headless mode
• replace silent=True with a verbose=False, to make it more obvious that setting it to print is not the default done thing
• repeat for plot_line
• hide the _save_plot method

[MichaelClerx]

Hmmm, no that doesn't let you modify before writing to disk

[MichaelClerx]

Closed in #57