GEOMESA-160 DWithin for non-point geometries

Removes assumption that DWithin geometry is a Point.
Let me see if I have this right...  Suppose I started with a lat-lon rectangle, selected three vertices, and formed a triangle.  The envelope for that triangle would be the entire rectangle, right?  And then we'd expand that further?  

I can see this as an incredibly simple and direct approach to getting DWithin to work with non-point geometries, but I'm wondering if we can come up another option.