Confirm vectorised constraint usage

(wtgee: Adding this comment from a stale (closed) issue):

I believe so, although we should probably open a separate, but related, issue, since at the moment I think you can actually create a ```Constraint``` with a vectorised list of constraints quite happily, but I'm not sure it broadcasts sensibly when used...

_Originally posted by @StuartLittlefair in https://github.com/astropy/astroplan/issues/189#issuecomment-550435883_

Start by locating the Constraint implementation and its existing tests, then inspect how vectorised lists of constraints are accepted and broadcast. Verify the behavior described in the issue and add or update coverage so the expected vectorised usage is explicit.

Written by the indexing model from the issue text.