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).