[PyROOT exp] long does not exist anymore in Python3, map to int

[PyROOT exp] Clear error indicator for fallback (enforced since py3.6)

[PyROOT] Use Python/C API utility for warnings

[PyROOT exp] Dont set error indicator during fetching keyword

[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

This is a cherry pick of the loader.py in cppyy-backend
version clingwrapper-1.7.2, which has the new treatment of
CLING_STANDARD_PCH and its setting to 'none' to prevent
the PCH check by Cppyy.

A complete update will come for the three packages of Cppyy
once the functionality of this PR:
https://github.com/root-project/root/pull/3579
is merged into TCling.

The issue is triggered when using the Python callable converter
of Cppyy.

The fix has been merged already in the CPyCppyy repo:
https://bitbucket.org/wlav/cpycppyy/pull-requests/18

Also, make sure CPyCppyy headers are in the PCH.

This commit contributes to fix the roottest-python-cling tests,
which use TPython, now part of CPyCppyy.

Instead of specifying the location of the PCH, we can just use
the magic word 'none' to tell cppyy not to check the PCH.
In both cases, the objective is to avoid a warning from cppyy
when importing it.

Make sure they are placed in ${build_dir}/include and
${install_dir}/include.

This is a pythonisation for TGraph, TGraph2D and their error
subclasses. In particular, it pythonises their getter methods
of the X,Y,Z coordinate and error arrays, which in C++ return
a pointer to a double.

The pythonization consists in setting the size of the array
that the getter method returns, so that it is known in Python
and the array is fully usable (its length can be obtained,
it is iterable).

Cppyy allows to add custom pythonisations for classes, but it does
not yet provide an API for adding custom type converters.

Until that API exists, we need to patch Cppyy with a converter
Python string -> TString, so that we do not have to create a
TString in Python when calling a C++ method that expects it.

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.

Throws IndexError if index is out of bounds and also makes it possible
to iterate over a TArray.

Throws IndexError if index is out of bounds and also makes it possible
to iterate over a TVector3.