Continuum fitting using individual points instead of spectral regions

Some of our JWST mid-IR spectra with low resolution have dense forests of emission lines (H recombination lines). The automated continuum fit tends to interpret as noise what is actually all emission lines, and there is often only a single point between the lines. To get a good continuum fit we really need to pass individual points rather than regions. Would be good if that were possible but I couldn't see it in specutils.

Start by locating specutils' automated continuum-fitting entry point and its existing tests, then trace how spectral regions are currently represented and passed into the fit. Done means callers can provide individual spectral points, with test coverage showing that dense emission-line spectra with few points between lines receive a usable continuum fit.

Written by the indexing model from the issue text.