TEST: Adding a test case with enough points for estimation, but not enough points for elevation support

ENH: Relaxing the constraints on the number of required points by explicitly disabling elevation if not enough points are provided

TEST: Removing the old DEMHandler tests. New, better ones are in Testing/Code/UtilitiesAdapter (and the tests should have been moved there anyway)

BUG: trigger UseGeoidIfNullFlag and force defaultHeightAboveEllipsoid to NaN to actually get the geoid used if null