Add a functional `write()` method to SpectrumList

[tbowers7]

The desired output of SpectrumList.write() would be a multiextension FITS file, where each extension contains the information from a single Spectrum1D object. The primary HDU could contain information about the number of included spectra, and possibly any common FITS keywords among the constituent Spectrum1D objects' metadata.

---

The current error message when one attempts to use the write() method is:

astropy.io.registry.base.IORegistryError: No writer defined for format 'tabular-fits' and class 'SpectrumList'.

The available formats are:

Format Read Write Auto-identify
------ ---- ----- -------------

[tbowers7]

Probably along the lines of this custom writer / loader, but less kludgy.