heterodyne: Retain merged spectra.
heterodyne: Initialise usedetpos in returned hash.

POINTING observations call _GET_MAKECUBE_PARAMS_POINTING_, but this
too does not set a value for usedetpos.  Later in calling primitives a
non-null usedetpos is needed to avoid a confusing MAKECUBE command
line.

This addresses Fault 20130504.001.