Factorize Computations in SARDEMProjection

SARDEMProjection uses several functions from SARSensorModel, which end up calling WorldToAzimuthRangeTime each, and which in turn:

• converts the WGS84 geopoint into an ECEF point (which is calculated again in the main filter when -withxyz is activated);
• searches for the zero doppler

(Of course, ideally OTB SARSensorModel should refactor these functions to permit to first search the 0 doppler from the ECEF point, then, provide functions that return the satellite position and/or velocities, the line sample YZ, and so on)