heterodyne: Get tau from ORAC_TAU header.
heterodyne: Remove units and label columns.
heterodyne: Switched a header being read, so revise documentation too.
heterodyne: Calculate and log a flat but do not apply it in legacy recipe.
heterodyne: New primitive to estimate RMS from the variance in spectral cubes

This is needed to complete the noise stats from the group cubes.
heterodyne: New primitive to determine median Tsys for spectral cubes

It also stores the value in FITS keyword MEDTSYS.

This statistic is needed to complete the noise statistics tabulation
for the group cube when MAKECUBE does not calculate it.
heterodyne: Call the nnew noise statistics primitives.
heterodyne: Minor tidying and augmented comment.