Add ITreeCodeOverride table

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

Update itree_code fetcher (itree_code_for_species_in_region) to use this logic:
* Look first for an i-tree code override in ITreeCodeOverride table (using in-memory version if supplied)
* If no override, find i-tree code in CODES dict as before

Also:
* Delete unused function get_codes_for_species()
* Conversion function _trees_for_eco() now used only locally
* Rename species_in_region() helper as has_itree_code(), and simplify
* Simplify language in plot_eco.html (fixes #925)
Merge pull request #946 from RickMohr/itreecode-table

Add ITreeCodeOverride table