SPICE reader: exclude HDUs for additional distortion information

[ebuchlin]

Solves #208 : read_spice_l2_fits failed on files with additional distortion information, which is stored in the WCSDVARR ImageHDUs. The code wrongly interpreted these as spectral windows HDUs, and this resulted in an error, as described in #208.

This PR (my first one in a SunPy affiliated project) is simply to ignore these HDUs containing distortion information.

Distortion from the instrument optics is already corrected in SPICE L2 FITS files, but the additional distortion in the WCSDVARR HDUs is the one that comes from spacecraft pointing changes, as obtained by the pipeline from the Solar Orbiter SPICE kernels.

In the future, a SPICE FITS file reader should have the option to take into account this information instead of ignoring it. How this should be done is still unclear, a possibility is to reproject on a non-distorted grid, but this has drawbacks, and requires deciding what to do in problematic cases (e.g. when pointing jitter in the scanning direction is larger than the raster step).

[DanRyanIrish]

HI @ebuchlin. Thanks so much for this PR. And congrats on making your first to a sunpy-affiliated package! These changes look good.

Could you please also add a changelog file in the changelog directory briefly describing this change? Instructions on how to write and name changelog files can be found here. All sunpy-sponsored packages use this way of building up changelogs so it's useful thing to know.

[nabobalis]

Thanks for the PR. I will get this merged and released today.

[nabobalis]

This is now on PyPI as 0.4.3.

[ebuchlin]

Thanks!

[DanRyanIrish]

Thanks @ebuchlin!