Make it easier to provide custom ground truth (Distribution) objects

[tlambert03]

As @melisande-c pointed out, it's rather clunky to create new sources of ground truth without them being "officially" recognized by microsim, and it's because of this union.

This could be fixed either via some base class that end users must subclass (which would be added to the union last), and could also probably be solved simply by adding a Protocol object to the union that detects whether the passed object implements render().