Create EntitySet field class, and ensure that EntityList and EntitySet can be used as keys

[mscott]

EntitySet? is basically EntityList? but using a set instead of a list.

As far as using either as a key: Schevo *should* be able to handle this fine, but as yet there are no unit tests that ensure correct operation in such circumstances. This ticket will fill in those missing gaps.

Also, add some documentation about field types in general.

[mscott]

implementation for EntitySet? and tests for EntityList? and EntitySet? as key are completed at r3180

[mscott]

some documentation at r3182

ready for review

[mscott]

Reviewed by pobrien. Merged at r3196.