AthenaPython: support GaudiConfig2 for PyAthena components
Implement proper support for GaudiConfig2 for the python components. The basic idea is to dynamically (on import) change the base class of of CfgPyAlgorithm, etc. This should be sufficient to support legacy and CA jobs, but will not work for jobs that try to mix both.
Also implement a trivial merge method that checks if two instances are identical. More complicated merge logics would have to be implemented in derived classes.
This also removes the need for special handling in the ComponentAccumulator.