Create tests for models of species, genus and family
Forest plots importer selects existing plots / species complex
Trees have dbh measurements, and through them, censuses
Census has a plot
Tests for dbh
Implement and test cn_measurement
Implement CN row importer
Rename BaseImporter to RowImporter
Use instance methods as opposed to static methods
Rename CsvDataImporter to BaseCsvHandler
Refactor BaseCsvHandler
Add some hooks for extending BaseCsvHandler
Remove superfluous private method
Rename ar_class to importer_class
Remove table_human_name requirement
Define a csv handler to read cn_curves