ExecutionContextImpl.findObject should have extra argument for case where we want to check inheritance below a particular class

[andyjefferson]

Being able to check the inheritance level of an "id" is essential, but on occasion we know that an object is of a particular level or subclass (e.g relation loading). We should be able to pass in the class. Perhaps this could even reuse the fourth argument, but would need to update calls to use it in that way, and fix javadocs to be clear about the usages.

[andyjefferson]

While doing this, we should consider adding a method ExecutionContext.findObjects for multiple ids of the same candidate.

[andyjefferson]

The latter item "ExecutionContext.findObjects" was added to v5.1.0.m5.
The original item needs a test case defining so that we have something to work against.