general: If a component in the chain does not exist that is not an error

If you ask to check for ".MORE.SMURF.EXP_TIME" the answer should be
false if ".MORE.SMURF" does not exist.

Ideally DAT__OBJNF would be exported by the perl NDF module.
heterodyne: Do not require EXP_TIME

If we are collapsing generic data cubes we do not really want
to mandate that .MORE.SMURF.EXP_TIME exists just so we can fill
in a FITS header.