Limit PulseheightFit choices for plate number from admin interface

Invalid values (0, 5, 6, etc) show up as '(None)' in the overview
and as no option selected in the model view.
Fixed typo.
Merge branch 'master' into tests
Added changing the ESD path when setting up the test datastore.
Added pulseheight fitting testing.
- Code cleanup
- Added fixture for histograms app.
Added more tests for pulseheight fit related functions.