ifu: Set frame of "dif" file to axis before plot

This is to prevent the problem mentioned in commit
7f1e6798d771e1941dfdda65272efd154c8ebdd8 where DISPLAY
raised an error regarding the default for the CENTRE
parameter.  On the first pass the WCS is derived from
the FITS airlock and didn't seem to be suitable.
Revert "ifu: Shortened product name to prevent DISPLAY anomalies."

This reverts commit 7f1e6798d771e1941dfdda65272efd154c8ebdd8.

Commit 72e20eddf9ce38d02945bea58baff78d11fd9387 is intended
to solve the DISPLAY problem.