Add custom loader for RC Spec

[dr-rodriguez]

We can consider adding a custom specutils loader for R-C Spec. This is not too difficult to do as long as we figure out what header keywords we can use to identify that the spectrum is indeed R-C Spec and not something else. We also want to find out which of the 4 flux arrays are the proper ones to use. This part of the header may be relevant:

BANDID1 = 'spectrum - background median, weights variance, clean yes'           
BANDID2 = 'raw - background median, weights none, clean no'                     
BANDID3 = 'background - background median'                                      
BANDID4 = 'sigma - background median, weights variance, clean yes'  

In the short term, we can add a custom loader to sedsimple.py. In the medium term, we can add it to AstrodbKit2 (which already has a custom Spex Prism loader). In the long term, this can be pull request to specutils.

[dr-rodriguez]

Started looking into this, but ran into some issues: R-C Spec has been used by both CTIO and KPNO and the header contents are different. Furthermore, sometimes the format of R-C Spec data from the same telescope is also different. This makes it very challenging to identify.

By default, the example I keep testing gets identified as a generic iraf spectrum. I have to force it to use the data loader which defeats the purpose.

I'll keep investigating.