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