Updates to target selection algorithms, mostly for SV

[geordie666]

Includes:

• First full implementations of QSO, LRG, ELG, and STD target classes for SV.
• Update and refactor of the MWS and BGS classes for the main survey.
• Change the name of the main survey MWS_MAIN class to MWS_BROAD.
• Augment the QA code to handle SV sub-classes such as the ELG_FDR_FAINT class.

@moustakas: It's always possible that changes to cuts.py will break the mocks. A few things to look out for:

• This PR addresses MWS-MAIN target definition and priority need to be changed #410, which you requested to be left open.
• The _prepare_optical_wise function now includes a dchisq return, which was needed for SV morphology checks (...dchisq, deltaChi2, brightstarinblob = _prepare_optical_wise(...).
• I refactored the main survey LRG selection, in a way that I think will work naturally with the mocks, but which could break inputs and outputs.

@apcooper: It'd be great if you could take a look at the new MWS selection code and check that I've addressed #410.

[apcooper]

The changes seem to be exactly what we asked for in #410, thanks.

[moustakas]

I've confirmed that these changes don't break select_mock_targets but I'll need to check the effect on (mock) target densities, etc., in a separate branch.

And @geordie666 thanks for the refactor in cuts which makes the code more modular and readable.