Function posAngRadians( ra1, dec1, ra2, dec2 )

Description:
Calculates the position angle between two points on the sky in radians. The result is in the range +/-pi. If point 2 is due east of point 1, the result is +pi/2. Zero is returned if the points are coincident.
Parameters:
ra1 (floating point)
right ascension of point 1 in radians
dec1 (floating point)
declination of point 1 in radians
ra2 (floating point)
right ascension of point 2 in radians
dec2 (floating point)
declination of point 2 in radians
Return Value (floating point):
bearing in radians of point 2 from point 1
Signature:
double posAngRadians(double, double, double, double)