Add function to add any bitmap to plot

[153957]

Allow easy plotting over an image. Can be used to plot locations over map tiles.

Todo:

• [x] Check in MultiPlot
• [ ] Easily set aspect ratio of plot to perfectly fit the image? (see #19)

[153957]

[davidfokkema]

You're on a roll, lately! I like it. I really, really, like it.

[153957]

Automatically setting the aspect ratio might be problematic, since we some kind of outer bound.. Does this include ticks?

e.g. for the Utrecht example the bitmap is 768x512 (= 3:2), however, the set Plot width and height ratio to get a nice fit are 7:5.