Clean of elevation management geoid parameter

Description

This issue is a follow up of #2198 In the context of the cars software, we are having troubles to unset the geoid parameter used in the otb applications.

CARS implements two homemade applications that use the elevation management:

• otbDEMReader used in a CARS API function called read_lowres_dem
• otbConvertSensorToGeoPointFast used for example in the CARS API function get_time_ground_direction

If several calls of the CARS API function are done in the same run session, the geoid parameter setting is not cleared between each otb application creation and execution. It seems that once m_GeoidFile is defined it is not unset when using DEMHandler::ClearDEM.

It might concerned usual otb application too but I couldn't print the geoid file path used by them (CARS also uses the StereoRectificationGridGenerator application).

Steps to reproduce

Two consecutive calls of the application using the elevation management (I only tried with the CARS API function described above), one with the geoid file set and one without. The m_GeoidFile should be both time set to the first geoid file.

Configuration information

Ubuntu, Centos, OTB 7.x (7.2), installation from binary version.