New SV selection for QSO with DR9

[Cyeche]

Tuning of the QSO SV selection for DR9
A few plots to illustrate the new selection

[geordie666]

Thanks, @Cyeche: This all looks good from the perspective of the code. I'll merge it later today.

[geordie666]

@Cyeche: I forgot, the usual requests apply for all changes to targeting cuts. Please:

(1) Update the wiki pages where appropriate with the new cuts.
(2) Update the changes.rst file in doc with the PR information.

I'll take care of (2) in my current working branch of desitarget, but please update the wiki.