spectroscopy: Use local $Frm for I with headers

The header reading (added in commit 07da7ed389) needs to happen
after the headers are copied into the polvec output file by
set_product (subroutine added in commit ff2449f884).  Also use
a local $Frm (instead of $tempFrm) so that _TAG_AS_REPRESENTATIVE_
operates on the correct file.