Unexpected schema change
Move code logic out to a code reader class

This class knows a lot about plot, tree and branch. As the codes start to
become more complicated I will move the logic for branch codes to Branch
for example.
Update toughness measurement to use new code reader
Generate branch architectures
Schema includes branch architectures
Update TODO
Tidy up find or create branch code
Define spec for Branch Architecture
WIP branch architecture migration
WIP branch architecture schema
Implement import and validation of branch architecture
Generate wood densities table
Define wood density spec
Define wood density logic