spectroscopy: Merge the headers of the new frame.

Without this step, only the current integration's headers are accessed.
Since these headers do not include the instrument name, it is not
possible to find the corresponding header-translation class.  Merging
with the observation headers fixes this.
CGS4: Use STATS instead of HISTAT to derive the median and mode.

This is to avoid determining the statistics using a too-sparse
histogram within HISTAT.