[BUG]: TEC calculation should use geocentric coordinates

[alfonsoSR]

Describe the issue:

According to version 1.1 of the IONEX standard, IONEX files provide TEC as function of geocentric latitude and longitude. As of version 1.2.11, pride uses the geodetic coordinates of the station.

How to reproduce the issue:

The source code associated with this bug is in delays.Ionospheric.load_resources(). In particular:

            # Station latitude and longitude for each coverage epoch
            coords = coordinates.EarthLocation(
                *baseline.station.location(tec_epochs, frame="itrf").T,
                unit="m",
            ).to_geodetic("GRS80")
            lat: np.ndarray = coords.lat.deg  # type: ignore
            lon: np.ndarray = coords.lon.deg  # type: ignore

OS and Python version:

NA

Expected behavior:

No response

Current behavior:

No response

Additional context:

The IONEX standard also provides guidelines on how to obtain the TEC through interpolation, and it mentions a "rotation" of the maps to take into account the position of the Sun. It would be a good idea to revise our implementation considering these suggestions.