Add optional pixel to mm conversion in plantcv.analyze.size and others

[rayn-alex]

Is your feature request related to a problem? Please describe.
I want to convert the pixel size values to mm so it's written in the outputs with the unit mm

Describe the solution you'd like
Add an option to the function that leads to a conversion of the px values to mm or mm². This could simply be a conversion factor. E.g.:
plantcv.analyze.size(img, labeled_mask, n_labels=1, label=None, px_conversion_factor=None)

Describe alternatives you've considered
It's always possible to do the conversion outside plantCV. However, I think it'd be convenient and it might be useful for more users.

If you all think an option like this makes sense, I could implement this in plantcv.analyze.size as well as all the other functions where this might be relevant (please help me to list all).