Add way to customise grid function plots within Jupyter Notebooks

[jahargreaves]

Plotting of Grid Functions within Jupyter Notebooks is done with plotly I believe, It would be great if there were some way to be able to use its features to customise the plots that bempp produces e.g. changing limits or adding colorscales. Options would be to: a) accept additional arguments and pass them to plotly; b) return a the plotly figure object, for subsequent editing by the user.

[jahargreaves]

PS: Also the mode flag (‘element’ or ‘node’) doesn't seem to make any difference to what is plotted. The plot follows the type of space, which makes sense. But is bodes the question of what this flag is intended to do?