Add: NetworkGroup.__add__ method. Fixes #200

This commit implements the `+` operator for the `NetworkGroup` object
and lets you "add" two `NetworkGroup` objects together.

```python
ng1 = NetworkGroup.load_json("ng1.json")
ng2 = NetworkGroup.load_json("ng2.json")

ng = ng1 + ng2
```

micone/main/network_group.py

@@ -66,6 +66,13 @@ def __repr__(self) -> str:
         n_contexts = len(self.contexts)
         return f"<NetworkGroup contexts={n_contexts} nodes={n_nodes} links={n_links}>"
 
+    def __add__(self, other: "NetworkGroup") -> "NetworkGroup":
+        """Combine two `NetworkGroup` objects and return a new `NetworkGroup` object
+        The new `NetworkGroup` contains nodes and edges from both the input objects
+        """
+        networks = [*self._networks, *other._networks]
+        return NetworkGroup(networks)
+
     def _combine_nodes(self, all_nodes: Dict[int, DType]) -> DType:
         """ Combine nodes of individual networks into a single list """
         nodes: DType = []