Add jump vector display

[andenacitelli]

It is often useful to be able to view jump vectors; this PR adds a keyword argument, plotJumps, to the plot() function, which causes it to plot all the jump vectors in a dark gray color (to avoid conflicts with the usual heatmap).

Example Output:

If you'd like me to open a PR to a different branch or for me to change anything, let me know.

[drlukeparry]

Thank you for this submission

In principle, I can include this perhaps as another option to show jumps between hatch vectors for better visualisation,rather than referring these globally as 'jump vectors'

Technically, due to my own implementation the order of scanning is not correct i.e. the hatch geometry then the contour geometry is collected and plotted separately rather than in the actual sequential order in the build file you would witness during scanning. I would like to correct this or provide an alternative plotting function for this and then incorporate the contribution.

[drlukeparry]

Implemented an improved sequential layer plotting visualisation. This includes jump vectors (in dash), which includes contour vectors too.

[andenacitelli]

Looks great - thanks!