Add VariableCategory, M2M from VariableDescription
Migration for adding VariableCategory
Make category name unique
Migration for unique variable category
Update load.py to populate variable categories
Add API View for Variable Categories

categories is m2m with variables.  The retrieve view for the category includes the variable descriptions
Include categories when retrieving a variable

Adds a serializer that includes the M2M categories, only used in the
retrieve single variable API view
Update load_all_datasets to include categories
Merge pull request #57 from NESCent/load_var_categories

Model and scripts to load Variable Categories