GEOMESA-201 Compute geometries from Geometry Filters

* This pr makes a first pass at extracting geometries to cover from the geometry filters.
* In order to do this, various things are refactored.
* GeohashUtils.getUniqueGeohashSubstringsInPolygon is widened to accept Geometries.
[picky] As this geometry is no longer strictly polygonal, perhaps we should use a different variable name? and change the output label on line 397?