Internalizing detabs

This version fixes a bug where the detector absorption was not included
in output background spectra or images.

In general, this caused a % level bias at low energies, but could be important for faint observations.

The background does not have to be refit with this update; only nuskybgd_image.pro and/or nuskybgd_spec.pro need to be rerun.

For nuskybgd_image.pro, DO NOT set the /noremakeinstr, as these files need to be remade (but once made for a given observation, the keyword can be used again).

Please report any issues to D. Wik.