[examples] Add python example to build a reduced robot model

examples/build-reduced-model.py

+import pinocchio as pin
+import example_robot_data
+import numpy as np
+
+# Goal: Build a reduced model from an existing URDF model by fixing the desired joints at a specified position.
+
+# Load UR robot arm
+robot = example_robot_data.loadUR()
+
+# Check dimensions of the original model
+print('standard model: dim=' + str(len(robot.model.joints)))
+for jn in range(len(robot.model.joints)):
+    print(robot.model.joints[jn])
+
+# Create a list of joints to lock
+jointsToLock = ['wrist_1_joint', 'wrist_2_joint', 'wrist_3_joint']
+
+# Get the ID of all existing joints
+jointsToLockIDs = []
+for jn in range(len(jointsToLock)):
+    if robot.model.existJointName(jointsToLock[jn]):
+        jointsToLockIDs.append(robot.model.getJointId(jointsToLock[jn]))
+    else:
+        print('Warning: joint ' + str(jointsToLock[jn]) + ' does not belong to the model!')
+
+# Set initial position of both fixed and revoulte joints 
+initialJointConfig = np.matrix([0,0,0,    # shoulder and elbow      
+                                1,1,1]).T # gripper)
+
+# Option 1: Build the reduced model including the geometric model for proper displaying of the robot
+robot.model, robot.visual_model = pin.buildReducedModel(robot.model, robot.visual_model, jointsToLockIDs, initialJointConfig)
+
+# Option 2: Only build the reduced model in case no display needed:
+# robot.model = pin.buildReducedModel(robot.model, jointsToLockIDs, initialJointConfig)
+
+# Check dimensions of the reduced model
+print('reduced model: dim=' + str(len(robot.model.joints)))
+for jn in range(len(robot.model.joints)):
+    print(robot.model.joints[jn])
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