Move importer classes into own folder
Rename traits tree to tree
The a tree's code is called tree_code

Not code
Trees actually belong to sub plots
WIP move importers
Arizona importer produces an import result
Use the new models appropriately
Tighten up the CodeReader logic
Update wood density to new style
Forest plots importer updated to new style
Trees must be defined before branches
Common importer test class
Common importer base class
Fix relations on some model specs