Fix gazebo sim rover

[Terracom12]

Gazebo sim rover is mostly just the diff-drive-skid template right now. Fix to align with the urdf changed in sam_bot_description.

The command ign sdf might help with conversions.

Ensure that GPS is also published with respect to the correct link.

[Terracom12]

ign sdf will work in part, but it seems that the definitions in the urdf file don't correspond very well with sdf. Might be simpler to manually define some of the joints in the diff drive sdf file.

See output below:

Warning [parser_urdf.cc:2771] Error Code 18: Msg: urdf2sdf: link[base_link] has no <inertial> block defined. Please ensure this link has a valid mass to prevent any missing links or joints in the resulting SDF model.
Error Code 18: Msg: urdf2sdf: allowing joint lumping by removing any <disableFixedJointLumping> or <preserveFixedJoint> gazebo tag on fixed child joint[base_joint], as well as ensuring that ParserConfig::URDFPreserveFixedJoint is false, could help resolve this warning. See http://sdformat.org/tutorials?tut=sdformat_urdf_extensions for more information about this behavior.
Error Code 18: Msg: urdf2sdf: allowing joint lumping by removing any <disableFixedJointLumping> or <preserveFixedJoint> gazebo tag on fixed child joint[gnss_moving_base_joint], as well as ensuring that ParserConfig::URDFPreserveFixedJoint is false, could help resolve this warning. See http://sdformat.org/tutorials?tut=sdformat_urdf_extensions for more information about this behavior.
Error Code 18: Msg: urdf2sdf: allowing joint lumping by removing any <disableFixedJointLumping> or <preserveFixedJoint> gazebo tag on fixed child joint[gnss_moving_rover_joint], as well as ensuring that ParserConfig::URDFPreserveFixedJoint is false, could help resolve this warning. See http://sdformat.org/tutorials?tut=sdformat_urdf_extensions for more information about this behavior.
Error Code 18: Msg: urdf2sdf: allowing joint lumping by removing any <disableFixedJointLumping> or <preserveFixedJoint> gazebo tag on fixed child joint[lidar_joint], as well as ensuring that ParserConfig::URDFPreserveFixedJoint is false, could help resolve this warning. See http://sdformat.org/tutorials?tut=sdformat_urdf_extensions for more information about this behavior.
Error Code 18: Msg: urdf2sdf: allowing joint lumping by removing any <disableFixedJointLumping> or <preserveFixedJoint> gazebo tag on fixed child joint[zed_camera_joint], as well as ensuring that ParserConfig::URDFPreserveFixedJoint is false, could help resolve this warning. See http://sdformat.org/tutorials?tut=sdformat_urdf_extensions for more information about this behavior.
Error Code 18: Msg: urdf2sdf: [9] child joints ignored.
Error Code 18: Msg: urdf2sdf: [9] child links ignored.
Warning [parser_urdf.cc:2774] Error Code 18: Msg: urdf2sdf: link[base_link] is not modeled in sdf.<sdf version='1.9'>
  <model name='sam_bot'>
    <frame name='base_joint' attached_to='base_link'>
      <pose>0 0 -0.319 0 -0 0</pose>
    </frame>
    <frame name='base_footprint' attached_to='base_joint'/>
    <frame name='gnss_moving_base_joint' attached_to='base_link'>
      <pose>-0.341 -0.2154 0.1115 0 -0 0</pose>
    </frame>
    <frame name='gnss_moving_base_link' attached_to='gnss_moving_base_joint'/>
    <frame name='gnss_moving_rover_joint' attached_to='base_link'>
      <pose>0.341 0.2154 0.1115 0 -0 0</pose>
    </frame>
    <frame name='gnss_moving_rover_link' attached_to='gnss_moving_rover_joint'/>
    <frame name='lidar_joint' attached_to='base_link'>
      <pose>0.5175 0 -0.011 0 -0 3.14159</pose>
    </frame>
    <frame name='lidar_link' attached_to='lidar_joint'/>
    <frame name='zed_camera_joint' attached_to='base_link'>
      <pose>0.4465 0 0.107 0 -0 0</pose>
    </frame>
    <frame name='zed_camera_link' attached_to='zed_camera_joint'/>
  </model>
</sdf>