Add Python support

[olaszsoma]

We would like to access Runaphys in python environment to test the runaway transport models. This requires the compilation of the code into dll files. This should be tested.

[pordaniel01]

DLL compilation can be achieved by adding the SHARED flag to the add_library function in CMakeLists.txt:
add_library(runin SHARED ${SOURCE_FILES})

[pordaniel01]

If you pass the -DBUILD_DLL=true to cmake, then .so file will be generated, which is the linux equivalent of a .dll file. In theory, it works the same way as a regular .dll file

[leferi99]

I cannot use the functions from the compiled .so library even though ctypes managed to import it into Python. I think this is caused by the fact that Python is written in C, so we need to wrap the C++ code in extern "C" { } as seen in the linked pages below:
straightforward tutorial
detailed Stackoverflow examples

[leferi99]

This one seems like a better tutorial
Also I started working based on this and I found that ctypes can create C structures from python which might make it easier to call functions from Python.

I managed to compile the code to the shared .so library with my modifications, now I'm testing if I can call the functions from Python.

[leferi99]

As I mentioned the code can be built, and I added a python file in the tests folder which also runs fine until the very end. At the point when the C++ function Runaphys_advance_runaway_population is called however I get a Segmentation fault. I think this is caused by the fact that data passing between Python and the C++ code is not too simple. I found a Stackoverflow thread and the ctypes documentation also mentions at the end of this paragraph that in certain cases a buffer should be used for passing data and pointers between languages.

[leferi99]

Also I used the ctypes.byref() method because it's supposed to be faster, but maybe the ctypes.pointer() method would be better. That method creates a python object.

[leferi99]

I asked a question on StackOverflow regarding the Segmentation fault problem. Also I found another Stackoverflow thread which might be useful. I made some modifications to some header files as well as the python file but I still get Segmentation fault.

[leferi99]

I've got a complete answer to my question and I applied the changes so now the test works for me.

[leferi99]

For the commit later today

If you want to build a shared library type:

mkdir build && cd build #creates the folder where the build files are created.

cmake .. -DBUILD_DLL=true && make

This will create Runaphys.so in build/src/

Currently the advance_runaway_population() function can be called from Python 3.x IF a shared library is built. To test this functionality after the shared library was built type:

cd ../test

python3 python_test.py

You should get a success or a fail indicator.