fix: adding loopjson encoder

LoopStructural/utils/json_encoder.py

@@ -0,0 +1,17 @@
+import json
+class LoopJSONEncoder(json.JSONEncoder):
+    def default(self, obj):
+        """All jsonable loop objects should have a tojson method
+
+        Parameters
+        ----------
+        obj : LoopStructuralObject
+            An object from loopstructural
+
+        Returns
+        -------
+        str
+            string representing the json encoding
+        """
+        return obj.__tojson__()
+