Add unique constraint to attributes

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Rough outline :

Create a custom exception, UniqueConttaintException

Create a UniqueConstraint class which can be added to models similar to Attribute. It takes a list of attribute names which must be unique.

Create a unique_constraint table to hold current vales with the class attribute name as one column and another with the current values in use (a list of tuples)

Add to the save object function to check and maintain those values and raise the exception as necessary.