Issue with imageToProximateImagingLocus() in the SAR model

[oleg-alexandrov]

It looks to me that the function imageToProximateImagingLocus() in the SAR model has the same issue as imageToRemoteImagingLocus(), which got handled in #361.

The current implementation of imageToProximateImagingLocus() does "tangent = cross(spacecraftVelocity, lookVec)", per https://github.com/USGS-Astrogeology/usgscsm/blob/dev/src/UsgsAstroSarSensorModel.cpp#L659. But that is not the same as the definition of this proximate locus, which says "the proximate imaging locus at a given ground point is the line tangent to the imaging locus at that point".

The "tangent" in the code is the cross-track direction, so it is in the sensor plane. But, since the imaging locus goes from a point in the sensor plane to the ground, the tangent to it also goes towards the ground, rather than being in the sensor plane.

In fact, given that the current SAR implementation assumes that the locus is a line, so all radar, atmosphere, and other physics are ignored, the proximate locus is the same as the locus itself.

If there's no objection, I propose modifying this "proximate" function to call the other one, imageToRemoteImagingLocus().