* In CRL::Cyclop/CMakeLists.txt, add *again* the MOC files to the list
    of .cpp . Don't know what is happening here with MOC under Qt 5.
* In CRL/python/helpers.io, cleanly fails if neither PyQt 4 nor PyQt 5 is
    found. And tell it directly because this the module tasked to handle
    the exceptions/errors...
* In Cumulus/plugins/AboutWindow.py, try PyQt 4 then PyQt 5.
* In documentation/UsersGuide, now tells explicitely that Qt 4 must be
    used under RedHat 7 and Qt 5 under Debian.