Feature request: Porting the RVSAO package from IRAF to specutils

I think at least the `bcvcorr`, `emsao`, and `xcsao` tasks from the RVSAO package in IRAF would be really handy if they are ported to Specutils. I would be happy to help adding the feature if none such is currently in place either in Specutils or other Astropy-affiliated packages, but will need some guidance at least to start.

More information regarding RVSAO can be found at [http://tdc-www.harvard.edu/iraf/rvsao/](http://tdc-www.harvard.edu/iraf/rvsao/).

Start by reviewing the Specutils repository and other Astropy-affiliated packages for existing implementations, then read the linked RVSAO documentation for the bcvcorr, emsao, and xcsao tasks. The issue names no files or tests; done would require an agreed scope and working Specutils ports of the selected RVSAO functionality.

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