PyCore: Re-implementation of embedded Python as a separate module (Major change)

This MR introduces a re-implementation of embedded Python as a new module. This is the second step of the major issue described in #223 (closed).

The Python-related parts of the codebase are gathered into a new module, PyCore. The new module provides the Python and Numpy C-API to the rest of the codebase, so that the other modules remain strictly Python-free.

Previously, the parts of the codebase which used Python C-API explicitly, were Base/Py, Device/Data/Datafield, and Sample/Multilayer/PyImport.

The ‘stable’ part of the Python C-API is marked explicitly in the embedded Python interpreter (see PyCore/Embed/PythonInterpreter.h). The compatibility across Python 3.7–3.9 is ensured; see https://docs.python.org/3/c-api/stable.html. Note that the Numpy C-API has no official guarantee for stability; see https://numpy.org/doc/stable/reference/c-api and https://numpy.org/doc/stable/reference/c-api/array.html .

Moreover, as the first step, the embedded Python can use the fabio Python package to read different file formats (see issue #156). This is yet to be tested thoroughly.

Further changes:

• The interface of MakeLib function is simplified to avoid global variables.
• SwigLib is cleaned up and improved.
• Tests for embedded Python are turned off and should be re-implemented.

Related to #223 (closed) and #156.

PyCore/Embed/PyObjectDecl.h

-// forward declarations needed for using `PyObject`
+//  ************************************************************************************************
+//
+//  BornAgain: simulate and fit reflection and scattering
+//
+//! @file      PyCore/Embed/PyObjectDecl.h
+//! @brief     Forward declarations needed for using `PyObject`.
+//!
+//! @homepage  http://www.bornagainproject.org
+//! @license   GNU General Public License v3 or higher (see COPYING)
+//! @copyright Forschungszentrum Jülich GmbH 2018
+//! @authors   Scientific Computing Group at MLZ (see CITATION, AUTHORS)
+//
+//  ************************************************************************************************
 
-#ifndef PYOBJECT_H
-#define PYOBJECT_H
+#ifndef BORNAGAIN_PYCORE_EMBED_PYOBJECTDECL_H
+#define BORNAGAIN_PYCORE_EMBED_PYOBJECTDECL_H
 
 #ifndef PyObject_HEAD
 struct _object;
@@ -10,4 +22,4 @@ typedef _object PyObject;
 
 typedef long int np_size_t; // size type used in Numpy
 
-#endif // PYOBJECT_H
+#endif // BORNAGAIN_PYCORE_EMBED_PYOBJECTDECL_H