odcombine, bad wavelength masks

- onedspec.odcombine now replaces onedspec.scombine because it respects
bad pixel ('wavelength') masks.
- finalize() was renamed to combinespectra().
- maskspectra() is a new function that creates bad wavelength masks for
all dispersion-corrected science (and standard star, if they exist)
spectra.
- maskspectra() also adds a keyword 'BPM' to the 1-header of the
dispersion-corrected science which it corresponds to; this 'BPM' header
keyword is used by odcombine.