Update QSO selection for SV, minor improvements to random catalog generation

[geordie666]

This PR:

• updates the QSO random forest for SV to facilitate different selections in the north and south.
• adds seeds to the random catalog generation for better reproducibility.
• defaults to using the DUST_DIR environment variable when generating randoms if a map directory is not passed.

[geordie666]

@CAClaveau and/or @Cyeche: Could you please check that this PR agrees with what you'd expect for your proposed updates to the QSO Random Forest selection in SV (the first commit of the PR)?

If this does look correct, could you please update the SV wiki page:

https://desi.lbl.gov/trac/wiki/TargetSelectionWG/SurveyValidation#QuasarsQSO22:MultivariateSelection

to reflect your new selection?

[geordie666]

@CAClaveau: After our conversation over email, I've made your requested corrections to the SV QSO selection code. Can you please check to see if this now looks correct?

[CAClaveau]

@geordie666: OK, everything looks correct, thank you.