#9058 Unit support

This should mostly cover unit conversion in webclient, see http://trac.openmicroscopy.org.uk/ome/ticket/9058.
To test it go to webclient and find image with multtimepoint and check if metadata shows 

```Z-sections/Timepoints: 	1 x 3 min 20 s```

Then choose image that shows:

```Pixels Size (XYZ) (mm): 	63.81 x 63.73 ```

Please test it with various images to make sure all convertions are handled properly

Main viewer should be tested parallel. Under the section Image information / Dimensions you should see exactly the same values and units.