ENH: improved fortran error message

[aburrell]

Requested feature and solution

If I can make one suggestion, it is to include the name "apex" or "IGRF" or something similar in the error message inside the COFRM routine of magfld.f90 when calling for an epoch outside the IGRF time span. That would be very helpful for finding the cause of the error in a large codebase, especially when it is going to be maintained by people who have not done the initial development. The currently used label "COFRM" did not immediately trigger an association with this package/functionality for me.

The problem or gap this feature will address

This will provide more informative errors to users