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.

Device/Data/Datafield.cpp

@@ -214,6 +214,9 @@ PyObject* Datafield::npArray() const
     Status init{PythonInterpreter::Numpy::initialize()};
     if (init.isError())
         return nullptr;
+
+    ASSERT(rank() <= 2);
+
     std::vector<size_t> dimensions;
     for (size_t i = 0; i < rank(); i++)
         dimensions.push_back(axis(i).size());
@@ -246,8 +249,6 @@ PyObject* Datafield::npArray() const
     } else if (rank() == 1) {
         for (size_t i = 0; i < size(); ++i)
             *array_buffer++ = (*this)[i];
-    } else {
-        return nullptr;
     }
 
     // returns a _new_ reference; ie. caller is responsible for the ref-count