Spectral indices

A use case I don't see obviously in place is a way to measure spectral indices, which are ratios of the flux in spectral regions. These can be defined in a number of ways, so it is probably beyond the scope to define them, but computing the mean, median, integrated flux in a region *with uncertainties* would be highly useful. I don't think lineflux covers this; is there a function to integrate spectral flux yet (if not, happy to put it in analysis)

Start by inspecting the existing lineflux functionality and the analysis package for spectral integration. Compare available mean, median, and integrated-flux operations and their uncertainty handling, then define a focused scope and tests for the selected behavior.

Written by the indexing model from the issue text.