Fix units if value is 0.0

To test:
 * select an image with more than one z-sections
 * go to the first z-sections. Make sure the units is 0.0 microns
 * Go to the projection tab. Make sure the units is correctly displayed e.g. 1.4-2.6 microns

See units in main view before and after
![units-before](https://f.cloud.github.com/assets/1022396/1732500/6b3fbc90-631f-11e3-8b0a-8a74556230f7.png)
![units-after](https://f.cloud.github.com/assets/1022396/1732502/6e1ba2c6-631f-11e3-94c3-cbf390cea9c1.png)

See units in projection view before and after
![unit-projections-before](https://f.cloud.github.com/assets/1022396/1732506/76990024-631f-11e3-96bb-3576d6339b05.png)
![unit-projection-after](https://f.cloud.github.com/assets/1022396/1732509/7d40455e-631f-11e3-99c4-b94d6ff00d58.png)