Allow renaming of class methods

[Thoemi09]

• add new method get_python_name to the worker class
  • checks if a given function declaration has a rename attribute and renames it if so
• use get_python_name for functions as well as class methods

[parcollet]

• To be rebased, I slighlty cleaned the implementaiton of the function in the other PR.
• Can we add a simple test for this ?

[Thoemi09]

There is already a PR in c2py: flatironinstitute/c2py#7