Adds new proofreading schema

[dxenes1]

• This new schema would be used for reference tables with the target_id as a foreign key to the nucleus id in nucleus_neuron_svm.
• Also adds 'roughly clean' and 'roughly extended' to the proofreading status enum to track neurons that have gone through some rounds of proofreading but are not comprehensively clean or extended.
• Let me know if you'd like test cases added before merging.

[fcollman]

Sorry for the delay.. couple comments. These schemas are identical, there is no need for that... you can have the same schema and just put two different tables with different table names and different descriptions, just give the schema a more generic name and you can make the table name more specific.

Second, you forgot to give the new schema an entry in the init.py dictionary of schemas, that's our mapping from schema string to python class.

[dxenes1]

Hi Forrest thanks for the review! Added the imports to the init.py file.

For the second class that is very similar to the first, we wanted a schema with no enum validation in the status_axon and status_dendrite fields to have a place to record our proofreading neurons with APL-specific proofreading status. I renamed this one GeneralProofreadingStatusReference to better fit its purpose. I can also move to a different PR or if there is a way to create a table with the CompartmentProofreadStatus and disable the validation that also works!