Fix scalebar units in PDF export

[will-moore]

Fixes #406

To test:

• Add scalebars with different units to different images
• Export as PDF
• Check that units are shown correctly in info page:

[jburel]

export option is disabled. is the export script on merge-ci?

[jburel]

tested using https://merge-ci.openmicroscopy.org/web/figure/file/64877

The feature works, but the order of the scale bars does not corresponds to the order of the display
scale bar of the second image is first in the list

[will-moore]

The images in a figure have no order (their position doesn't correspond to their order).
I collect the various scalebar lengths from all the images in the figure and only show each unique value, so it's likely that each scalebar applies to many images in different locations.
This export of scalebar lengths in the PDF was really needed before we had the ability to add labels to scalebars on the panels. Now that labels are supported (and will be the default with #413) the display of lengths in the PDF is not so important - and not worth spending too much time on improving (this is a bug-fix only).

[jburel]

thanks for the clarification. Merging