Add ITreeCodeOverride table

ITreeCodeOverride table
* Stores "species + region --> itree code" overrides
* Declares that a species/region pair must be unique

Update itree_code_for_species_in_region()
* Take species and region instead of otm_code and region_code
* If region missing, get instance's default region
* Look first for an i-tree code override in ITreeCodeOverride table
* If no override, find i-tree code in CODES dict as before
* Simplify callers

Also:
* Delete unused function get_codes_for_species()
* Rename species_in_region() helper as has_itree_code(), and simplify
* Simplify language in plot_eco.html (fixes #925)