Refactor `GenericImportEvent.validate_main_file` and callers
Handle UDFs for tree imports

* Include tree and plot UDFs as legal field names for tree imports
* Validate UDF columns from imported CSV file
* Column name has model prefix, e.g. "Density" -> "Tree: Density"
Remove dead fields from exporter
Merge pull request #1796 from RickMohr/tree-validation-uses-udfs

Tree validation uses udfs