Function midLon( lon1, lon2 )

Description:
Determines the longitude mid-way between two given longitudes. In most cases this is just the mean of the two values, but this function copes correctly with the case where the given values straddle the lon=0 line.
Parameters:
lon1 (floating point)
first longitude in degrees
lon2 (floating point)
second longitude in degrees
Return Value (floating point):
longitude midway between the given values
Examples:
Signature:
double midLon(double, double)