See dave_electrical_mating.world for an example.  Allowing arbitrary
names should make it possible to use the plugin for multiple
plug-socket pairs.  The plugin still pairs one socket to one plug,
but it should be ok to use the same socket or plug with multiple
plugin instances.

The plugin is associated with the socket model.  Functionality
is unchanged, and the plugin utilizes a "plug" model from the
SDF world.

Conversion will (hopefully) make the plugin easier to use and
incorporate its capability into models using Xacro.

Plug and socket plugin updates to clean up the code and to base
the alignment on the world pose of the plug and socket links.  This
should make it easier to add plug/socket pairs to arbitrary models.

The platform model is just a box for now, but a better model will
be incorporated once a mesh is available.

Updated alignment and joint functionality to work when the socket
is mounted to another model and not aligned with the world X axis
(previous version was using RelativePose when checking the alignment
of the plug and socket).

Also, changed the prismatic joint function to allow it to slide
more reliably before it is locked.

Alignment check now compares the pose of the socket and plug links
more directly in a single function.  Alignment is checked in 3 steps:
  Orientation: plug & socket link frame orientations match
  Lateral: plug position is along the socket link X axis
  Distance: plug is within 0.5 meters of the socket link

If all three checks pass, the proximity check (which is unchanged)
is used to determine when to create the prismatic joint.

Replaced ROS messaging with Gazebo messaging (there's no other ROS
dependency, so it's really just a Gazebo plugin).  Changed "verbose"
variables to a DEBUG define instead.  Changed the demos to set verbose
to true when launching Gazebo so that the plugin info messages are
printed to stdout (screen).

Also fixed a bug that kept the joint from being locked correctly.

The error was because the joint was being deleted but not
"removed" from the model when the plug was detached from the
socket.  This caused an error on subsequent attempts to
plug into the socket.

Also did some code cleanup and updated the models so that they
are not so bright on most hosts.