polar stereographic coordinate transformation (map to lat/lon)

The National Snow and Ice Data Center (NSIDC) and Scientific Committee
on Antarctic Research (SCAR) use a version of the polar stereographic
projection that Matlab does not have. This file does transformations to
geographic coordinates from map coordinates to facilitate
comparisons with other datasets.

POLARSTEREO_INV transforms map coordinates to lat/lon data for a polar stereographic system
[LAT,LON]=POLARSTEREO_INV(X,Y,EARTHRADIUS,ECCENTRICITY,LAT_TRUE,LON_POSY)
LAT and LON are in decimal degrees with negative numbers (-) for S and W.
X and Y are the map coordinates (scalars, vectors, or matrices of equal size).
EARTHRADIUS is the radius of the earth defined in the projection
(default is 6378137.0 m, WGS84)
ECCENTRICITY is the earth's misshapenness
(default is 0.08181919)
LAT_TRUE is the latitude of true scale in degrees, aka standard parallel
(default is -70). Note that some NSIDC data use 70 and some use 71.
LON_POSY is the meridian in degrees along the positive Y axis of the map
(default is 0)

Equations from: Map Projections - A Working manual - by J.P. Snyder. 1987
See the section on Polar Stereographic, with a south polar aspect and
known phi_c not at the pole.

See also: POLARSTEREO_FWD