xytraceset in quicklook

[julienguy]

Replace all instances of quicklook.qlpsf.PSF by in quicklook , modify read_xytraceset and add write_xytraceset.
This PR fixes the issue #681 .

[julienguy]

I realize my description of the pull request was not intelligible, sorry for that. Let me try again:

• I replaced everywhere in quicklook the use of the class qlpsf.PSF by the class desispec.xytraceset which is used in the offline pipeline. xytraceset is a container for the trace coordinates and the PSF sigma values.
• I added write_xytraceset in desispec.io.xytraceset
• This is to make sure we use only one set of IO routines for spectral traces in desispec, in order to avoid inconsistencies as reported in issue New PSF format fails QL #681.
• This turned out to touch a lot of files in quicklook.

[rstaten]

I have tested this branch on arcs and science and no longer receive the errors we were previously seeing. Thanks for this fix.

[sbailey]

Thanks for testing @rstaten. Merging now.