geopotential 0.3.8

Earth's gravity field from spherical harmonics: read ICGEM .gfc models, then compute gravity, geoid, gradients and water height at a point or on a map, stable to degree 2700.
Documentation 
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# Test fixtures

Real files, kept whole up to `end_of_head` (or the header) and truncated after it, downloaded on
2026-10-23, plus one synthetic file.

| File | Source | Kept | Licence / credit |
|---|---|---|---|
| `costg_2026-07_n10.gfc` | COST-G GRACE-FO July 2026, [ICGEM](https://icgem.gfz.de/getseries/02_COST-G_/Grace-Grace-FO_RL02.1/GSM-2_2026182-2026212_GRFO_COSTG_BF01_0201.gfc) | degree ≤ 10 | CC BY 4.0, doi:10.5880/COST-G.ICGEM_02_L2 |
| `swarm_style_synthetic.gfc` | Synthetic: the header layout of ESA's Swarm EGF_SHA_2 files with invented coefficients. ESA's data terms (clause B.5) allow passing Swarm data on only to recipients who accept them, so real Swarm data is not stored here | 8 records | MIT, like the code |
| `goco06s_n4.gfc` | GOCO06s, [ICGEM](https://icgem.gfz.de/tom_longtime) | degree ≤ 4 | CC BY 4.0, doi:10.5880/ICGEM.2019.002 |