Write down formal spec for LISdf

[williamshen-nz]

We can also autogenerate docs. Though our code will need more docstrings for that to be useful

[vacancy]

Using this thread to track newly created tags (i.e., tags not supported/listed in the original SDF format).

<gui>
  <camera name="default_camera" definition_type="lookat">
    <xyz>3 6 3</xyz>
    <point_to>0 6 1</point_to>
  </camera>
</gui>

The attribute "definition_type" can take two possible values ("pose" or "lookat")

<include>
  <scale>0.8</scale>
</include>

The scale value can be either a single float or three values, denoting the scaling factor for x, y, and z axes.

[vacancy]

The lisdf definition supports the feature.

<group name="left-arm">
  <joint name="xxx" />
  <link name="xxx" />
  <group name="left-arm-gripper" />
  <chain base_link="xxx" tip_link="yyy" />
</group>

The standard is based on http://wiki.ros.org/srdf