[feature idea] scale bar

[swelborn]

Add a scale bar to the images.

This would be another bqplot ImageGL mark. Would have to be generated with prior knowledge of pixel scale/binning of image.

For ex. if pixel width is 20 nm and binning is 2, each pixel would be 40 nm. This would come from image metadata and/or scan parameters.

[swelborn]

Added something like this using a bqplot.Line. Can be found commented in tomopyui.widgets.plot. Not implemented.