- Sky, Sigma spectra separate FITS files

- New function copySciSkySig() copies/extracts the 3- and 4-bands in the
flux-calibrated spectra such that the sky and sigma spectra are now in
separate files.
- The purpose for that is that odcombine does not combine associated
band spectra, only the primary spectrum.
- 'RUSIGMA' header keyword replaced with 'RUAPSUM'
- 'RUSIGMA' now used to identify the sigma spectrum with its
flux-calibrated science spectrum.
- 'RUSKY' has a similar use for sky spectra
- The copySciSkySig() function has been made available in the individual
tasks menu.
- Further testing may be required for the combination of the sky and
sigma spectra at the end of the combinespectra() function.