[python] Avoid VirtualFunction in python interface

• Remove include of python/function.hh from python-related headers. This uses the deprecated VirtualFunction interface and thus clutters the compile output with many warnings. To keep the dunepythontest.cc working, the include is now added there directly.
• Implement conversion to std::function and BoundarySegment
• Switch dunepythontest.cc from VirtualFunction to std::function. This 'test' mostly demonstrates the use of the interface. This is now done in terms of std::function instead of the deprecated PythonFunction which implements the deprecated VirtualFunction interface.