-Improve error messages for missing or non-car initial object

-Add skip for MetaDrive tests
-Update test_pedestrian_movement to have specific coordinates for Car and Pedestrian

src/scenic/simulators/metadrive/__init__.py

@@ -1,8 +1,8 @@
-metadrive = None
 try:
     import metadrive
 except ImportError:
-    raise ImportError("Metadrive is required. Please install the 'metadrive' package.")
-if metadrive:
-    from .simulator import MetaDriveSimulator
-del metadrive
+    raise ImportError(
+        "Metadrive is required. Please install the 'metadrive' package (and sumolib) or use scenic[metadrive]."
+    )
+
+from .simulator import MetaDriveSimulator

src/scenic/simulators/metadrive/model.scenic

@@ -31,7 +31,6 @@ from scenic.simulators.metadrive.simulator import MetaDriveSimulator
 from scenic.simulators.metadrive.actions import *
 from scenic.simulators.metadrive.behaviors import *
 from scenic.simulators.metadrive.utils import scenicToMetaDriveHeading
-from metadrive.utils.math import norm
 
 
 if globalParameters.get("sumo_map") is None:

src/scenic/simulators/metadrive/simulator.py

@@ -97,11 +97,16 @@ def __init__(
         sumo_map_boundary,
         **kwargs,
     ):
-        # NOTE: MetaDrive requires at least one agent to be defined per simulation run
+        # NOTE: MetaDrive requires at least one agent to be defined per simulation run,
+        # and the first object must be a car (to serve as the ego vehicle).
         try:
             if len(scene.objects) == 0:
                 raise SimulationCreationError(
-                    "The Metadrive interface requires you to define at least one Scenic object."
+                    "Metadrive requires you to define at least one Scenic object."
+                )
+            if not scene.objects[0].isCar:
+                raise SimulationCreationError(
+                    "The first object must be a car to serve as the ego vehicle in Metadrive."
                 )
         except SimulationCreationError as e:
             print(e)

tests/simulators/metadrive/test_metadrive.py

@@ -2,7 +2,13 @@
 
 import pytest
 
-from scenic.simulators.metadrive import MetaDriveSimulator
+try:
+    import metadrive
+
+    from scenic.simulators.metadrive import MetaDriveSimulator
+except ModuleNotFoundError:
+    pytest.skip("MetaDrive package not installed", allow_module_level=True)
+
 from tests.utils import compileScenic, pickle_test, sampleScene, tryPickling
 
 
@@ -110,8 +116,8 @@ def test_pedestrian_movement(getMetadriveSimulator):
             do WalkForward() for 2 steps
             do StopWalking() for 2 steps
 
-        ego = new Car
-        pedestrian = new Pedestrian with behavior WalkThenStop
+        ego = new Car at (30, 2)
+        pedestrian = new Pedestrian at (50, 6), with behavior WalkThenStop
 
         record initial pedestrian.position as InitialPos
         record final pedestrian.position as FinalPos