Warn about non-hook objects in pysipp_conf.py files

[goodboy]

There should be a validation step which warns about any python object (function or other) inside a pysipp_conf.py file which isn't found registered with the plugin manager. The plugin.mng.is_registered() meth can be used for this purpose. This should help with catching misnamed hook functions.

PS I don't think we want to actually prevent the user from defining their own custom objects hence my suggestion for a simple warning.