Read headers of temporary Frame for _TAG_AS_REPRESENTATIVE_.

Commit 8405d80938b6003 added calls to the product_id() method,
which attempts to access a header.  In some primitives this was
a temporary Frame which had not read in the headers from the NDF
and lead to "Use of uninitialized value $subsys in concatenation (.)
or string..." messages appearing in previously working recipes.
heterodyne: Prevent QA failing due to all receptors failing test.

Forgotten that I'd not committed this fix from January 23.
heterodyne: Issue warning when the pixel size is enlarged.

Jan was puzzled why the pixel size was different from expected.
At least not be secretive.  Jan was also checking the criterion used
to define the minimum pixel size.