Add base classes for FunctorKeys that return reference objects
Documentation for FunctorKey base classes.
Add memory-manager getter to BaseRecord

This will be necessary to support FunctorKeys that return arrays.
Update docs on aggregates to use new field delimiter convention
Add FunctorKey for arrays
Add methods to ArrayKey to create new fields in Schemas
Use two template arguments in FunctorKey invocation

This lets us support implicit conversion in FunctorKey-based set.
FunctorKey-based const reference access should be a const method.
Add comments to ArrayKey test.
Merge branch 'u/jbosch/DM-836'