Add order trace file to header as a keyword

[awhoward]

@RussLaher or @bjfultn seem like the right person to handle this.

This will be helpful information for the QLP.

[RussLaher]

The information is already contained in the header indirectly of the master flat. For example,

$ imheaders -i kpf_20240206_master_flat.fits | grep ORD ORDRMASK= '/data/reference_fits/kpf_20230730_order_mask_untrimmed_made20230804&'

The 20230730 observation date in the order-mask filename corresponds to the order trace on same observation date.

I will make this explicit in the header with ORDTRACE keyword giving the filename in the header of the master flat.