Optional physics features (#62)

* Add and use entityCast helper function

* return early for missing features

* move minimum map out of entityCast

Signed-off-by: Louise Poubel <louise@openrobotics.org>
Signed-off-by: Nate Koenig <nate@openrobotics.org>

CMakeLists.txt

@@ -85,7 +85,7 @@ ign_find_package(ignition-physics1
     mesh
     sdf
   REQUIRED
-  VERSION 1.6)
+  VERSION 1.7)
 set(IGN_PHYSICS_VER ${ignition-physics1_VERSION_MAJOR})
 
 #--------------------------------------

src/systems/diff_drive/DiffDrive.cc

@@ -293,8 +293,11 @@ void DiffDrivePrivate::UpdateOdometry(const ignition::gazebo::UpdateInfo &_info,
       this->rightJoints[0]);
 
   // Abort if the joints were not found or just created.
-  if (!leftPos || !rightPos)
+  if (!leftPos || !rightPos || leftPos->Data().empty() ||
+      rightPos->Data().empty())
+  {
     return;
+  }
 
   this->odom.Update(leftPos->Data()[0], rightPos->Data()[0],
       std::chrono::steady_clock::time_point(_info.simTime));