Added timer action and simiplified mp-400 urdf to enable reliable multirobot simulation

[VineetTambe]

Simulating multiple robots (more than 3) was highly unreliable as spawning multiple nav stacks and robots caused a high CPU spike causing gazebo crashes and missed spawns. To resolve this - the following changes were made:

1. Add "TimeAction" to the launch file with a configurable delay between the launches - this launches the navstack and spawn actions with a set time delay in between enabling reliable launch of multiple robots (tested with 15 robots on neo_track1)
2. Simplified collision mesh to reduce CPU computation