evselect_spectrum doesn't support grouping spectra

[DavidT3]

Currently it is not possible to generate grouped spectra, as many people do. This should be added as an option in evselect_spectrum.

In the case where someone requests a grouped spectrum, I can just add a specgroup command on the end of the stack, and overwrite the spectrum file.

[DavidT3]

evselect_spectrum now supports grouping spectra with the SAS specgroup command. The default is to require 5 counts minimum per channel, but users can also specify over sampling and a minimum signal to noise.