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/PyCore.h

@@ -16,7 +16,6 @@
 #error no need to expose this header to Swig
 #endif
 
-#ifndef USER_API
 #ifndef BORNAGAIN_PYTOOLS_PYCORE_H
 #define BORNAGAIN_PYTOOLS_PYCORE_H
 
@@ -55,4 +54,3 @@
 
 #endif // BORNAGAIN_PYTHON
 #endif // BORNAGAIN_PYTOOLS_PYCORE_H
-#endif // USER_API