Loader for STIS echelle data (potential feature request)

This is probably something that should live in ST software, but putting it here so we can make a decision about it. There was a user request from @jkcarlberg to have an adapter that will take in STIS echelle data, and spit out a `Spectrum1D` object. In this specific example, @jkcarlberg would like to have something that can read the units stored in the table column.

Start by determining whether this belongs in specutils or ST software, then review the project's existing loader entry points and how Spectrum1D objects are created. Done means an agreed adapter for STIS echelle data that reads units from the table column and returns a Spectrum1D object.

Written by the indexing model from the issue text.