Add lerp planning_example

This commit adds all the files that are needed to run the
[lerp_planning_example](https://ros-planning.github.io/moveit_tutorials/doc/creating_moveit_plugins/plugin_tutorial.html?highlight=lerp)
example.

config/lerp_planning.yaml

@@ -0,0 +1 @@
+num_steps: 40

launch/demo_lerp.launch

@@ -0,0 +1,6 @@
+
+<launch>
+  <include file="$(dirname)/demo.launch">
+    <arg name="pipeline" value="lerp"/>
+  </include>
+</launch>

launch/lerp_planning_pipeline.launch.xml

@@ -0,0 +1,22 @@
+<launch>
+
+  <!-- LERP Plugin for MoveIt! -->
+  <arg name="planning_plugin" value="lerp_interface/LERPPlanner" />
+
+  <!-- The request adapters (plugins) used when planning with TrajOpt.
+       ORDER MATTERS -->
+  <arg name="planning_adapters" value="default_planner_request_adapters/AddTimeParameterization
+				       default_planner_request_adapters/FixWorkspaceBounds
+				       default_planner_request_adapters/FixStartStateBounds
+				       default_planner_request_adapters/FixStartStateCollision
+				       default_planner_request_adapters/FixStartStatePathConstraints" />
+
+  <arg name="start_state_max_bounds_error" value="0.1" />
+
+  <param name="planning_plugin" value="$(arg planning_plugin)" />
+  <param name="request_adapters" value="$(arg planning_adapters)" />
+  <param name="start_state_max_bounds_error" value="$(arg start_state_max_bounds_error)" />
+
+  <rosparam command="load" file="$(find panda_moveit_config)/config/lerp_planning.yaml"/>
+
+</launch>