measurements should contain an additional column with image calibration unit

[dlegland]

follwoing suggestion from @tischi, to facilitate interpretation of measurements, it is necessary to know whether numeric values are returned in "pixel" units or in calibrated units (mm, µm...).
One simple solution to help the user would be to add an extra column to measurements table, containing the unit name used for measurements, or "pixel"/"voxel" if image was not calibrated (or if measurement did not take into account spatial calibration).

see https://forum.image.sc/t/morpholibj-measurements-output/62614