v2.6 Changes

Features:

+ 20191229 minor changes to igrflib_v2.pro to extend data range to GUFM1/IGRF13
           coefficient range [1590-20205)

Bug fixes:

+ 20191229 small change to aacgm_v2.pro (moved definition of aacgm_v2_datetime
           to location after out-of-bounds check in order to make error msg
           consistent with that of DLM.)