Minor cleanup

* Clarify code in `diff_from_species`
* Remove unused return value from `validate_required_fields`
Create i-Tree code overrides

* Add helper `Species.get_itree_code`
* `diff_from_species` compares i-Tree codes
* `commit` creates i-Tree code overrides
* Refactor tests
squash to field reorg
wip
squash with prev
Stopgap change in how "choice" fields are processed

The importer's code for validation of choice fields used `settings.CHOICES`,
an OTM1 variable not present in OTM2. So we need a new strategy.

For trees the choice fields will be UDF's so we'll use their choices.

For species we need a solution for "Flowering Period" and "Fruit or Nut Period".
For now I've hard-coded the USA's 4 seasons, but we need a solution that supports i18n.
wip