JSON Serializer for pystencils config

This MR adds a custom JSON serializer to allow pystencils configs to be used as parameters in databases. Useful in parameter studies when using the more modern way of setting up simulations, i.e., using pystencils' CreateKernelConfig.

Can be extended in the future for other custom classes if needed.

conftest.py

@@ -65,6 +65,7 @@ try:
 except ImportError:
     add_path_to_ignore('pystencils/runhelper')
     collect_ignore += [os.path.join(SCRIPT_FOLDER, "pystencils_tests/test_parameterstudy.py")]
+    collect_ignore += [os.path.join(SCRIPT_FOLDER, "pystencils_tests/test_json_serializer.py")]
 
 try:
     import islpy