Adding prismatic motor type

[Daniel Angelov]

Is your feature request related to a problem? Please describe.
X-Y gantries are defined by 2 prismatic motors. Looking through the Hardware -> Add Motor tab, I can see BLDC, DC, AC, etc motor types. Though it becomes impossible (unless I am missing something) to model such a robot?

Describe the solution you'd like
The feature request would be a way to define prismatic motor types and be able to export a urdf like

<joint name="${prefix}axis1_joint" type="prismatic">
    <parent link="${prefix}tower"/>
    <child link="${prefix}axis1"/>
    <origin xyz="0 -0.2 0.3 rpy="0 0 0"/>
    <!-- Here we define the axis along which the geometry is moved -->
    <axis xyz="0 0 1"/>
    <limit effort="1000.0" lower="0.0" upper="2.0" velocity="0.1"/>
</joint>

Describe alternatives you've considered
An alternative is to potentially model the linear prismatic joint as a motor on a rail?

edit: This can be done when performing: Define Joints 0> Joint type -> Prismatic (rather than resolute)