10/2019
  this is apex code with IGRF 12 coefficients (released 2015)
  examples of how to use the code are given in test.f
  to test the code:
  compile test.f and apex.f90, run it, and compare the standard output with
  test.out
  
1/2020
  update the code with IGRF 13 coefficients (relases Dec 2019)
  https://www.ngdc.noaa.gov/IAGA/vmod/igrf.html

12/2024
  update the code with IGRF 14 coefficients (released 19th Nov 2024)
  https://www.ncei.noaa.gov/products/international-geomagnetic-reference-field

05/2025
  Update to add the ability to read the IGRF coffecients from the file provided by NGDC at 
  https://www.ngdc.noaa.gov/IAGA/vmod/coeffs/igrf14coeffs.txt.  This will be come the default 
  behavior with the next update of IGRF.  

  Also changes the license to BSD-3 clause.  
  
  The igrf14coeffs.txt file is included in the repository for ease of use, but that file is provided 
  by the US Government and is in the public domain and not covered by this license or copyright.