Update World Magnetic Model from WMM2020 to WMM2025

[zourzouvillys]

Summary

The World Magnetic Model (WMM) is the standard model for navigation, attitude, and heading systems. It is updated every 5 years by NOAA's National Centers for Environmental Information (NCEI) and the British Geological Survey (BGS). WMM2020 expired at the end of 2024, and WMM2025 was officially released on December 17, 2024.

This PR updates the entire library from WMM2020 (epoch 2020.0, valid 2019–2024) to WMM2025 (epoch 2025.0, valid 2024–2029).

What changed

Coefficients (pkg/wmm/)

• bindata.go — Regenerated the embedded, gzip-compressed WMM.COF binary asset with the WMM2025 spherical harmonic coefficients (degree/order 12, 91 coefficient pairs + secular variation). The file size changed from 4557 → 4554 bytes.
• testdata/WMM2025.COF — New coefficient file downloaded from NOAA NCEI. Header: 2025.0 WMM-2025 11/13/2024.
• coefficients.go — Updated the doc comment to reflect the new default file validity period (11/13/2024 – 12/31/2029).

Error model (pkg/wmm/magnetic_field.go)

Updated global average uncertainty constants from the WMM2025 technical report:

Component	WMM2020	WMM2025	Unit
X	131	137	nT
Y	94	89	nT
Z	157	141	nT
H	128	133	nT
F	148	138	nT
I	0.21	0.20	deg
D (const)	0.26	0.26	deg
D (1/H)	5625	5417	nT

The declination uncertainty formula remains sqrt(errDA² + errDB²/H²).

Tests (pkg/wmm/*_test.go)

• coefficients_test.go — Added TestGetWMM2025Coefficients: validates 7 representative (n,m) pairs at epoch and epoch+1 year, matching the COF file exactly.
• magnetic_field_test.go — Added TestAll2025TestValuesFromPaper: validates all 100 official NOAA test points spanning 2025.0–2029.5, at altitudes from 0–98 km, covering all 7 main field components (D, I, H, X, Y, Z, F) and their secular variation rates. Tolerances: ±0.05 nT for field values, ±0.005° for angles.
• testdata/WMM2025_TEST_VALUES.txt — Official test values from the NOAA WMM2025COF distribution.
• All existing WMM2015v2 and WMM2020 tests continue to pass (they load their own COF files).

Documentation & CLI

• README.md — Updated coefficient download URL (now points to NCEI), model year references (2020 → 2025), and package description.
• cmd/wmm_point/main.go — Updated usage strings, model year references, and comment header.

Build

• go.mod — Added Go module file (github.com/westphae/geomag) to enable building with modern Go toolchains. The repo previously relied on GOPATH mode which is no longer the default.

Test plan

• go test ./pkg/wmm/ -run TestGetWMM2025Coefficients — all 56 coefficient assertions pass
• go test ./pkg/wmm/ -run TestAll2025TestValuesFromPaper — all 100 NOAA test points pass (1400+ field component assertions)
• go test ./pkg/wmm/ -run TestGetWMM2020Coefficients — existing WMM2020 tests still pass
• go test ./pkg/wmm/ -run TestAll2020TestValuesFromPaper — existing WMM2020 test values still pass
• go test ./pkg/wmm/ -run TestGetWMM2015v2Coefficients — existing WMM2015v2 tests still pass
• go test ./pkg/wmm/ — full wmm package test suite passes
• Manual verification: compare wmm_point output against NOAA's online calculator for a known location

Data source

All coefficient and test data downloaded from the official NOAA distribution:
https://www.ncei.noaa.gov/sites/default/files/2024-12/WMM2025COF.zip