Add fix for ground to image calculation for LRO NAC cameras (#341)

* Add fix for ground to image calculation for LRO NAC cameras

* Make the choice of the sign of m_iTransL only once at model loading

* Reword a comment

* Robustness fix, must always ensure fractional part is 0.5

* Remove the abs, cannot assume those values are positive

* Added opposite flight and detector test

Co-authored-by: Jesse Mapel <jmapel@usgs.gov>