Add some test functions to ppigrf

[santisoler]

Add tests against precomputed IGRF values. There are small differences between the results that ppigrf generates and the ones that the NCEI Calculator produces on dates that require some interpolation along the dates. The difference is not significant.

[santisoler]

@klaundal I've wrote some tests that compare ppigrf results against a precomputed grid I got from the NCEI Calculator. I'm seeing some discrepancies between them. Do you have any idea why it might be happening? It's due to floating point precision? Maybe a different set of coefficients? Would love to hear your opinion on this.

[klaundal]

@santisoler I did a quick comparison for one point with the NOAA calculator now. The deviations are very small, but too large to be floating point precision I think. I doubt that the coefficient files are different. One possibility is that it is an issue with how the time interpolation between model coefficients is implemented. And/or the conversion between geodetic and geocentric coordinates and components (I get a very good match in the eastward component, which is not affected by this).

Still, the deviations are tiny, typically less than 1 nT. This rarely has any practical consequence, but it would be good to know why it happens. Comparison with an open code would be preferable though.

[santisoler]

I suspect you're right and this is coming from the interpolations on the dates. I also checked ppigrf against outputs of igrf (which I suspect the NCEI calculator is running on the background) and I get the same differences. I couldn't generate a grid with igrf because their code was failing due to some Xarray error (probably they were using a different version of Xarray). But I checked with some individual points and the differences are still there.

I've just run a quick check comparing on 2020-01-01 (so no interpolation should be carried out) and the tests pass. So apparently the difference is coming from the date interpolation. I will push new tests with no interpolation on date.

[santisoler]

Hi @klaundal! After a few weeks (time flies!) I managed to fix the tests so they pass. I've increased the atol in the second case (in which we have to interpolate along the dates). I think this is sufficient for now and it should be ready to be merged. Let me know what you think!