Use CaseGroup/CaseName to refer to hydsim cases

`CaseGroup/CaseName` is established syntax used in various UIs, Optimal
Gateway, and Automation Framework. As discussed in
Volue/energy-sim#742 (comment).

There's an argument for more user friendly error handling here, but I
think this is API compatible with a future improvement, and I want to
get this API break done ASAP. For now you'll get a `ValueError` if the
split doesn't split.

Closes #398.

src/volue/mesh/_connection.py

@@ -379,7 +379,6 @@ def update_rating_curve_versions(
         def run_simulation(
             self,
             model: str,
-            case_group: str,
             case: str,
             start_time: datetime,
             end_time: datetime,
@@ -390,7 +389,9 @@ def run_simulation(
             if start_time is None or end_time is None:
                 raise TypeError("start_time and end_time must both have a value")
 
-            simulation = f"Model/{model}/{case_group}.has_OptimisationCases/{case}.has_OptimisationParameters/Optimal.has_HydroSimulation/HydroSimulation"
+            case_group, case_name = case.split("/", maxsplit=1)
+
+            simulation = f"Model/{model}/{case_group}.has_OptimisationCases/{case_name}.has_OptimisationParameters/Optimal.has_HydroSimulation/HydroSimulation"
 
             request = core_pb2.SimulationRequest(
                 session_id=_to_proto_guid(self.session_id),