Show units

Use custom units and digits in field display and editing
* Add helper functions to units.py
* Make sure plot & tree fields have "in request.instance" so they'll use custom units and digits
* Format display values client-side on form save (digits, units)
* Ignore .orig files (generated by kdiff3)
* Display eco benefits values using configured units and digits

(Unit tests pending, but posting for Mike's use)