Add support for FITS file creation

[ehsteve]

We should add support for FITS file creation to complement the existing functionality for CDF files. This is required to support the other "half" of heliophysics missions which use FITS files instead of CDF files. This means enabling creating, reading, and validating FITS files. These files are primarily used by solar observatories and usually for image data. There are a number of existing standards for FITS files (see the FITS standards page. Astropy already provides the ability to read and write FITS file and some light support for validation (see docs). Heliophysics add some other requirements on FITS (see reference section in Solarnet recommendations).

For validation for FITS file metadata standards see this pull request to SunPy. For test files consider the test files used by SunPy (here], though note that very few headers are currently solarnet compatible.

[Alrobbertz]

Solar Orbiter Paper: https://www.aanda.org/component/article?access=doi&doi=10.1051/0004-6361/201935305
No slides found here: https://dash.heliophysics.net/ihdea2023/ihdea.html