Use DNF for specifying selection filters

[sidneymau]

Problem & Solution Description (including issue #)

This allows us to specify the selection filters using disjunctive normal form; e.g.,

Selections:
    gold: [[[LSST_obs_i, <, 25.5]]]
    blend: [[[LSST_obs_i, <, 26.0]]]
    everything: null

Code Quality

• [ ] My code follows the code style of this project
• [ ] I have written unit tests or justified all instances of #pragma: no cover; in the case of a bugfix, a new test that breaks as a result of the bug has been added
• [ ] My code contains relevant comments and necessary documentation for future maintainers; the change is reflected in applicable demos/tutorials (with output cleared!) and added/updated docstrings use the NumPy docstring format
• [ ] Any breaking changes, described above, are accompanied by backwards compatibility and deprecation warnings