Improvements for the URDF xsd specification

[aminya]

The improvements include specifications for:

• joint types
• link
• image
• camera
• laser ray
• ray
• sensor
• gazebo

This builds on top of the XSD changes from #183 and then fixes the remaining XSD issues related to:

• Link's element repetition specific
• gazebo's lax processing
• the robot namespace backward compatibility

The specification can be tested via the following (e.g. in VsCode with the XML extension):

<?xml-model href="https://raw.githubusercontent.com/aminya/urdfdom/xsd/xsd/urdf.xsd"?>
<robot name="test">

The above code is backward compatible with the parsers as it doesn't add a namespace to the robot tag. If a namespace is added, some parsers like Python's parser add ns0: before the tags. Not requiring a namespace makes it possible to verify the XML without changing the tags.