polar stereographic coordinate transformation (lat/lon to map)

POLARSTEREO_FWD transforms lat/lon data to map coordinates for a polar stereographic system
[X,Y]=POLARSTEREO_FWD(LAT,LONG,EARTHRADIUS,ECCENTRICITY,LAT_TRUE,LON_POSY)
X and Y are the map coordinates (scalars, vectors, or matrices of equal size).
LAT and LON are in decimal degrees with negative numbers (-) for S and W.
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)

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
map coordinates from geographic coordinates to facilitate
comparisons with other datasets.

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_INV