AGIPD: handle bad pixel constants with three dimensions with more cells than requested

The exception that started https://redmine.xfel.eu/issues/177981 seems to stem from another place where assumptions are made about the number of cells in a constant matching the number of cells we asked for. This merge request gets rid of that assumption - as long as we get at least as many cells as we asked for, all is good. Also, including fix for addon schema override related issue I just spotted because it got in the way of testing (and should be deployed ASAP, too).