Support name attribute for collisions

[nlamprian]

I added the name attribute in Collision, which interestingly enough, was only present in Visual.

[jspricke]

@clalancette could we get this merged? Thanks!

[nlamprian]

Is someone maintaining the package? If so, please review the PR. As it is, specifying a name in a collision tag appears as an error that confuses.

[sloretz]

Context:

• The urdf schema allows a name attribute on collisions, but is commented as undocumented: https://github.com/ros/urdfdom/blob/99c6f58120a5dfd1f9fc36419cd9f9aa0246efde/xsd/urdf.xsd#L146
• urdfdom_headers urdf::Collision type has a name field: https://github.com/ros/urdfdom_headers/blob/8ec38993c21c6913602b9519ca07e4e018af88b5/urdf_model/include/urdf_model/link.h#L183
• urdfdom does parse and populate the name attribute on a Collision: https://github.com/ros/urdfdom/blob/99c6f58120a5dfd1f9fc36419cd9f9aa0246efde/urdf_parser/src/link.cpp#L408-L410

Since the C++ API supports this, it seems like a good idea to me to add it to the Python API

[brinij]

@clalancette Could you please review this PR so it can be merged soon? That would be really helpful! Thanks!