[PyROOT exp] Move implementation of DeclareGenericCallable to separate source file

[PyROOT exp] Add feature to jit Python callable using numba

So far, the jitting is only supported for basic types.

[PyROOT exp] Add proxy class to switch between numba and generic impl

[PyROOT exp] Ensure no leaking PyObject if stepping out early in callable proxy

[PyROOT exp] Allow user to pass optional name of wrapper function

[PyROOT exp] Add test for dispatcher of wrapper functions

[PyROOT exp] Put in first round of comment

 - numba_only, generic_only and verbose flag
 - put only one decorator
 - change namespace of C++ side functions
 - modularize code
 - adapt tests

[PyROOT exp] Add missing DECREFs when stepping out due to an exception

[PyROOT exp] Remove unnecessary import of headers

[PyROOT exp] Make verbose=True to new default