This commit introduces the logic to display ROOT graphics when
using Python interactively. For that purpose, hooks are installed
to process the GUI events and to update the display.

Note that the threading approach is not used anymore: no thread
is explicitly created to process the events. The code based on
hooks should work on all the platforms, but it needs to be
tested. For the same reason, some code that handled the
threading case on Windows has been removed for now, and will be
restored only if necessary.

Most of the code has been picked from several parts of the
current ROOT.py and put together in the configuration of the
PyROOT TApplication.

The activation of the graphics is triggered only if ROOT is not
in batch mode. Such mode can be set by the user right after
importing ROOT.

The RPyROOTApplication is a TApplication that sets up the nuts
and bolts for interactive ROOT use from Python, closely
following TRint.

Variables are detected as used initialized if initialization is done via
a call to the interpreter.

The warning is introduced starting with gcc8 (cast-function-type).

Register keyword is deprecated since C++11. Observed with Python 2.7
installation and AppleClang/gcc7 in combination with C++17.

Set missing initializer in list, warning seen with gcc 7 and 8.

NULL conversion warnings appearing with gcc 7 and 8.

The fix is taken from the official CPyCppyy repository (commit 590cc73).

These tests will work again once this PR is merged:
https://github.com/root-project/root/pull/3640

The feature implemented by the PR consists in automatically adding
using declarations of methods from a base class to the derived
class. After the PR is merged, the feature will be provided by
the bindings.

This corresponds to the fix from Sergei Linev (1b565f24), which will
be integrated in cppyy-backend when the ROOT of Cppyy is updated to
a patch release of 6.16.

In the meantime, this patch reminds of the necessary fix.

In its last update (CPyCppyy 1.6.4), Cppyy automatically adds to
a derived class the overloads of a base class that come from
using declarations.

Therefore, that makes the pythonisation for plotOn redundant,
since everything will be handled at the bindings level.

[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.