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.

pystencils_tests/test_json_serializer.py

+"""
+Test the pystencils-specific JSON encoder and serializer as used in the Database class.
+"""
+
+import numpy as np
+import tempfile
+
+from pystencils.config import CreateKernelConfig
+from pystencils import Target
+from pystencils.runhelper.db import Database, PystencilsJsonSerializer
+
+
+def test_json_serializer():
+
+    # create pystencils config
+    config = CreateKernelConfig(target=Target.CPU, function_name='dummy_config', data_type=np.float32)
+
+    # create dummy database
+    temp_dir = tempfile.TemporaryDirectory()
+    db = Database(file=temp_dir.name, serializer_info=('pystencils_serializer', PystencilsJsonSerializer))
+
+    db.save(params={'config': config}, result={'test': 'dummy'})