This class represents a topographic coordinate. It has the exact same interface as DtVector. The intent was to keep DtTerrainReasonerAlgos decoupled from VRF code base, for the purpose of testing it outside VRF. This can be easily undone by replacing DtTopoPoint with a typedef, thus making it a mere alias of DtVector.
Note: in practice, testing outside VRF only proved useful for the hilltops finding algorithm. Position finding algorithms rely heavily on line-of-sight checks (not easily stubbed) and thus they were never tested outside VRF.