FUNCTION: dipole_offset PURPOSE: Calculate the location of the dipole using the IGRF coefficients and their secular variation. IGRF coefficients are the coefficients used in the spherical harmonic expansion of the earth's magnetic field and there is a standard formula for converting the first few of these coefficients to dipole offset. This version is accurate for years 1995-2000 Returns a 3-element vector in kilometers. INPUT: Optional argument is 4-digit year Algorithm by: Mike Temerin Written by: J.Rauchleiba 4/8/97