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.

GUI/View/Project/PyImportAssistant.cpp

@@ -104,12 +104,12 @@ std::unique_ptr<MultiLayer> code2sample(const QString& snippet, const QString& f
 {
     QApplication::setOverrideCursor(Qt::WaitCursor);
 
-    void* result_ptr = PyInterpreter::createMultiLayerFromPython(
-        snippet.toStdString(), funcName.toStdString(), bornagainDir());
+    void* pObj = PyInterpreter::pyscript2object(snippet.toStdString(), funcName.toStdString(),
+                                                "MultiLayer", bornagainDir());
 
     QApplication::restoreOverrideCursor();
 
-    return std::unique_ptr<MultiLayer>{reinterpret_cast<MultiLayer*>(result_ptr)->clone()};
+    return std::unique_ptr<MultiLayer>{reinterpret_cast<MultiLayer*>(pObj)->clone()};
 }
 
 //! Lets user select a function name which generates a MultiLayer.