calc_elev_corr
Called by: correct_ttime for each phase if elev_corr_on=1 and crust_corr_on=0.
Includes: iscloc.h, jb_model.h
Input arguments: Pointer to a phase structure.
Structure members updated: None.
Return: Travel time correction in seconds for elevation of station.
Calculates the extra time spent by a phase to traverse the crust
between the geoid and the surface. Does so by dividing the station elevation
by the upper crust velocity for the model in use and by a factor that accounts
for non vertical incidence.