[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

Cppyy does not automatically add to a class the method overloads that
come from a 'using' statement. For this reason, a pythonisation is
needed for RooDataHist to see the overloads it is using from
RooAbsData.

TClass::DynamicCast returns a void* that the user still has to cast.
This pythonisation provides the requested binding if the cast succeeded.

This feature is able to adopt memory from PyObjects defining an
__array_interface__ as RVecs.

The new interface in Converters deals with multi-dimensional arrays.

TPython::CPPOverload_Check does an additional check for the
initialization of the Python interpreter that we don't need.

this allows to create files with this factory pattern

myFile = ROOT.TFile.Open("myfile.root")

the pythonizations are three:
1. A C++ pyz to provide attr syntax. For example:

myfile.mydir.mysubdir.myHist.Draw()

2. A C++ pyz to allow users to invoke the templated method TDirectory::WriteObject<T>. For example:

h = ROOT.TH1F("h","",10, 0, 1)
myfile.WriteObject(h)

3. a Python pyz which implements TDirectory::Get in terms of the pythonized attr syntax

these are internal Cppyy utilities. In principle it would be better to
expose them in cppyy and use them in PyROOT and not duplicate the code.
This commit at least avoid to duplicate the code only once.