Add base_footprint

[mukmalone]

https://github.com/therobocademy/Module_1_ROS_for_Beginners/issues/7

<xacro:macro name="footprint" params="wheel_props">

<!-- base_footprint is a fictitious link(frame) that is on the ground right below base_link origin -->
<link name="base_footprint" />

<!-- Joint to connect base_footprint with base_link -->
<joint name="base_footprint_joint" type="fixed">
  <parent link="base_footprint"/>
  <child link="base_link" />
  <!-- origin xyz="0 0 ${wheel_radius - base_z_origin_to_wheel_origin}" rpy="0 0 0" /-->
  <!-- origin xyz="0 0 ${wheel_props['wheel']['radius'] - wheel_props['wheel']['z_offset'] + 0.1}" rpy="0 0 0" /-->
  <origin xyz="0 0 ${wheel_props['wheel']['radius'] - wheel_props['wheel']['z_offset']}" rpy="0 0 0" />
</joint>

</xacro:macro>