Add IGRF-14

[smithara]

Do not merge yet - Waiting final confirmation on the coefficients

With the new release we should make sure to communicate the impact of DGRF and validity periods.

• IGRF-13 covers 1900-2025 but is only definitive up to 2015.0 (includes the DGRF for 2015.0)
• IGRF-14 covers 1900-2030 but is definitive up to 2020.0 (includes the DGRF for 2020.0)
  • Thus IGRF-13 and IGRF-14 "disagree" after 2015.0

Since this PR changes the default model used to IGRF-14, it affects reproducibility (as compared with the IGRF-13 release) for dates after 2015.0

[smithara]

This PR changes the default IGRF used to IGRF-14 and simply includes IGRF14.shc in addition to the existing IGRF13.shc and makes them importable so that the user can switch between them like:

import ppigrf
from ppigrf.ppigrf import shc_fn_igrf13, shc_fn_igrf14

...

# Will produce same results as ppigrf version 1:
Be, Bn, Bu = ppigrf.igrf(lon, lat, h, date, coeff_fn=shc_fn_igrf13)
# Will produce different results from ppigrf version 1 (after 2015):
Be, Bn, Bu = ppigrf.igrf(lon, lat, h, date, coeff_fn=shc_fn_igrf14)
Be, Bn, Bu = ppigrf.igrf(lon, lat, h, date)   # default behaviour uses shc_fn_igrf14

IGRF-14 is now released and has a DOI now: https://doi.org/10.5281/zenodo.14012302 ... the NOAA and BGS online calculators are not yet updated but should be by the end of the year