heterodyne: Expand the noisestats table

Add additional columns for the UT start time, observation number,
exposure time, airmass, tau, and the RMS units and label.

Insert a hash comment symbol at the start of the column names.
Also add a comment giving the creation time of the log.

At present the format is such that column names align with their values,
but we may need to switch to CSV in the future for better machine
analysis.
heterodyne: Indent emission-finding code normally.
heterodyne: Add APPLY argument to decide whether or not to apply the flat.
heterodyne: New primitive to log flatfield ratios and their errors.
heterodyne: Use the new flat statistic log primitive.

Some additional information was collected first.
- Record the reference receptor and the observation numbers of
  contrubiting observations used to estimate the flatfield ratios.
- Calculate the errors on the ratios.