Phocube Slope and Normal Calculations (#4605)

* First pass at slope in phocube

* Added slope and azimuth cals to phocube.cpp

* Added local normal and ellipsoid normal back plan calculations to phocube

* Force pulled upstream new phocube main.cpp

* Removed unused includes and moved a few lines around

* Fixed up the includes one more time

* Updated phocube docs based on PR feedback

* Added changelog entry

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