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.
The features vary in most of the tests (the first 2 use the same features). I prefer to have each test be a stand-alone though. If we re-use the data stores between tests, then the result of each test can be affected by the one before it.