GEOMESA-169 fixing density iterator, restoring expansion of density map

There is still more work to be done on 169 to fix some artifacts in certain queries, but this is a first step that restores the expansion of features and fixes iterating over the sub-iterator and compiling the results.
Sure, sure.  I'm hoping we can 'engineer' our tests a little more...  I feel like the code to get a test data store is duplicated in many places.  Some standardization might help; that's what I'm hoping for.