Attempt to fix dictionary for edm::AssociationVector

Andrea Rizzi reported a problem reading edm::AssociationVectors from pyroot. The problem was a cache internal to edm::AssociationVector was not getting cleared on each event. An ioread rule was added for the class, however it appears there is a bug in ROOT which prevents this rule from being used.

This pull request sets up unit tests for edm::AssociationVector to prove it works properly in the full framework. There is also a commented out test for reading in pyroot which can be used to test any fixes in ROOT for the problem.