CPAlgorithms: add protections to EventSelection algorithms when running with missing inputs
Event selectors that run on lepton collections should be allowed to run on only electrons or only muons, but not neither. This MR implements this at the C++ level in the relevant algorithms, but also offers error messages in the high-level python configuration to catch wrong settings.
This is needed for the AMG PHYSLITE tutorial next week, so tagging as urgent.