Nadir-pointing Type

[ptgrogan]

When computing ground track geometries, TAT-C should specify whether the nadir-pointing observation is geocentric (i.e., satellite looks at the geocenter) or geodetic (i.e., satellite looks at the surface normal point below) and potentially add an option for the user to specify which type to use.

Currently, TAT-C uses the Skyfield wgs84.geographic_position_of function to project a satellite position to latitude/longitude/altitude and then zeroes out the altitude to obtain the sub-satellite point which is used as the center of ground track footprints. In other words, the current implementation is geocentric.