Add plot function for streak images

[jlaehne]

It would be nice to have a convenience plot function for streak camera images that plots an integrated spectrum on top and an integrated transient to the right of the map (e.g. lum.plot.plot_streak_image):

Brought up in #205 (comment)

The basic functionality could be extended to allow passing certain ranges for the two axes and show only the spectrum/transients for these ranges.

Eventually, even an interactive functionality with spanROIs could be implemented.