The user might use utilities which print on cout and expects the output
to be shown immediately.

This patch automatically flushes std::cout after each execution of a wrapper.

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).

Fixes ROOT_GENERATE_DICTIONARY not finding v7 headers.

Otherwise, find_file(headerFile) will claim "I did that search already".

(cherry picked from commit 180a22a53c7ee76d5059c086dac4c8972b75a563)

Without DISPLAY, everything might work just fine because no
TGClient is needed. But then do not spoil the party by trying
to display the popup!

This is pointless / almost never works, and is often done too eagerly.
Instead, let the TGClient do it (through TSystem) when actually needed.
The complaint about ssh -Y is moved there, so remove it here.

Co-authored-by: Jan Knedlik <j.knedlik@gsi.de>

Co-authored-by: Jan Knedlik <j.knedlik@gsi.de>

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.