Add very basic PhotoPipe selection (including z-band)

[elvinpoole]

added PhotoPipe selection v1 (_ppv1)

Closes #229

[cailmdaley]

Reviewed for the project board — the change itself looks good (the two-band OR is the right semantics for keeping objects with a measurement in either z-band column). A few things before merge:

Merge conflict — the Mask class has moved. The restructuring PRs #197/#199 dissolved run_joint_cat.py; Mask now lives in src/sp_validation/masks.py on develop. The three code hunks re-apply there mechanically (apply() and create_descr() are unchanged apart from formatting), and the new config file is untouched by the restructuring — so the rebase should be quick.

Smaller points, take or leave:

• The second column is found by a hardcoded self._col_name + "2" — worth a line in the Mask class docstring (its allowed-kind list also doesn't mention not_equal_2bands yet), or passing the second column explicitly.
• The FITS-header description "special, not equal to OR mask" is opaque in an output header; something like f"!={self._value} in {self._col_name} or {self._col_name}2" would let a reader reconstruct the cut.
• Nits in the config: "photometry cuts cuts ugri" typo + trailing whitespace on that line.

— Claude on behalf of Cail.