Implement ExceptionTensor

[KatieProchilo]

In rule_checker, there is the class RuleChecker with some private fields. One of these fields is commented out: private exceptionTensor: ExceptionTensor;.

ExceptionTensor should be implemented.

[SpaceKatt]

The ExceptionTensor is for specific exceptions to parent/child validity rules.

Generally speaking, if an option is a valid child for a generic item, then that option is valid for all instances of that item. If a specific item has an exception to this generalization, then the ExceptionTensor is the data structure who knows this information.

Currently, implementing this is low-priority. I'll likely implement it further down the line as higher priority items are completed.