Deprecate direct construction of Config objects

[bpiwowar]

To make things more explicit, configurations should be used either explicitely as a configuration object or not, i.e. use MyConfig.C(...) rather than MyConfig. This is handled by the method __new__ in Config at the moment.

In the future, this should be forbidden (i.e. the __new__ should be removed), but meanwhile it would be good to generate a warning when __new__ is called directly. The end-user documentation should also be updated.

Tasks:

• Add a warning in __new__
• Update the documentation