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/Sample/ImportMultiLayer.cpp

+//  ************************************************************************************************
+//
+//  BornAgain: simulate and fit reflection and scattering
+//
+//! @file      PyCore/Sample/ImportMultiLayer.h
+//! @brief     Implements createMultiLayerFromPython.
+//!
+//! @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)
+//
+//  ************************************************************************************************
+
+#ifdef BORNAGAIN_PYTHON
+
+#include "PyCore/Sample/ImportMultiLayer.h"
+#include "PyCore/Embed/PyCore.h"
+#include "PyCore/Embed/PyInterpreter.h" // BornAgain::importScript
+#include "auto/Wrap/swig_runtime.h"     // for creating a core object from Python
+#include <stdexcept>
+
+void* PyInterpreter::pyscript2object(const std::string& script, const std::string& functionName,
+                                     const std::string& typeName, const std::string& path)
+{
+    PyObjectPtr tmpModule{PyInterpreter::BornAgain::importScript(script, path)};
+
+    // locate the `get_simulation` function (it is an attribute of the module)
+    PyObject* pAddFn = PyObject_GetAttrString(tmpModule.get(), functionName.c_str());
+    if (!pAddFn)
+        throw std::runtime_error(errorDescription("PyInterpreter::BornAgain: "
+                                                  "Cannot locate the compiled function '"
+                                                  + functionName + "'"));
+
+    // create a `MultiLayer` Python object via calling the function
+    PyObject* ret1 = PyObject_CallFunctionObjArgs(pAddFn, NULL);
+
+    // clean up
+    Py_DecRef(pAddFn);
+
+    if (!ret1)
+        throw std::runtime_error(
+            errorDescription("Failed execting Python function '" + functionName + "'"));
+
+    // Construct a C++ object from the Python object.
+    // This conversion requires a SWIG-produced Python API.
+
+    void* argp1 = 0;
+    swig_type_info* pTypeInfo = SWIG_TypeQuery((typeName + "*").c_str());
+
+    const int res = SWIG_ConvertPtr(ret1, &argp1, pTypeInfo, 0);
+    if (!SWIG_IsOK(res)) {
+        Py_DecRef(ret1);
+        throw std::runtime_error(
+            errorDescription("Function " + functionName + " did not yield MultiLayer instance"));
+    }
+
+    return reinterpret_cast<void*>(argp1);
+}
+
+#endif // BORNAGAIN_PYTHON