Update SDF to include sensor links

[steph-lheureux]

The old SDF does not describe the sensors as <link>s, and as a result, the coordinate transforms do not appear in /tf. Update the SDF to include these sensors as links.

A thruster link which appears as a transform looks like this:

<joint name='thruster_joint_1' type='fixed'>
      <pose relative_to='base_link'>0 0 0 0 0 0</pose>
    <parent>base_link</parent>
    <child>thruster1</child>
    <axis>
    <xyz>0 0 -1</xyz>
    </axis>
</joint>
<link name='thruster1'>
    <pose relative_to='thruster_joint_1'>0 0 0 0 0 0</pose>
    <inertial>
    <pose>0.32378531987640202 -0.28832105189077301 -0.017377450240958998 0 0 0</pose>
    <mass>0.0077399991027584901</mass>
    <inertia>
        <ixx>2.20644758396638e-06</ixx>
        <ixy>9.80149056457269e-11</ixy>
        <ixz>-1.65242619966251e-11</ixz>
        <iyy>2.2079718651242701e-06</iyy>
        <iyz>-7.1929062949609399e-12</iyz>
        <izz>3.2148319855682202e-06</izz>
    </inertia>
    </inertial>
</link>

The sensors looked like this:

<frame name='imu_targetpoint1_joint' attached_to='base_link'>
    <pose>0 0 0 0 0 0</pose>
</frame>
<frame name='imu_targetpoint1' attached_to='imu_targetpoint1_joint'>
    <pose>0 0 0 0 0 0</pose>
</frame>