save the trace shift offsets in the psf file

[segasai]

This is the proposed followup from #2386 (assuming it's merged)
I propose to save the trace-shift offsets (in wavelength) in the psf file, to allow monitoring/improvement/debugging.
The extension/column names are somewhat temporary/can be changed if needed.
The increase in the file size from this is tiny ~ 5% for ~ 1Mb sized psf files.

One could in principle make this optional, i.e. save the offsets if some command-line argument is passed.

[coveralls]

coverage: 30.072% (-0.01%) from 30.086%
when pulling 9867229 on write_trace
into 083ca2b on trace_fix_improvements.