heterodyne: Read header into temp Frame object

Needed because the product_id() method expects a header to exist
whereas simply creating a temporary object and calling the file()
method won't populate the header part of the Frame.

This fixes:

    Use of uninitialized value $subsys in concatenation (.) or string
      at /Users/timj/work/oracdr/src/lib/perl5/ORAC/JSAFile.pm line 228.

when creating the Frame integ images.