Multi-mesh visualization support and rviz plugin improvements

[Schmluk]

Added support for multiple meshes and transparency for mesh visualization. Thought this feature might also be useful for others. Changes in particular include:

• Added a MultiMesh message that simply wraps the normal voxblox_msgs::Mesh for multiple publishers. This also ensures compatibility with earlier versions since the original definitions remain untouched.
• Added a multi_mesh_rviz plugin (which is almost identical to the mesh_rviz plugin) that supports multiple mesh instances in the same topic, similar to visualization_msgs/marker IDs.
• Added an ogre material based on 'BaseWhiteNoLighting' that allows to set alpha properly. Although this functionality is also available for the standard voxblox_msgs::Mesh it is not used there since the message does not contain an alpha channel (could be added if requested though).
• Tested with two to ~100 independent mesh layers as in the image below, which did not work using only one voxblox_msgs::Mesh topic, and where one of the layers is partially transparent.
• Added some quality of life improvements for both plugins:
  • Can now properly show/hide the meshes without discarding everything.
  • Can change the fixed frame after mapping is finished.

Replaces PR #327 that got a bit too corrupted to clean up properly.

image:

[ethzasl-jenkins]

Can one of the admins verify this patch?