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

[padhupradheep]

Wow!! Thanks very much for the update Vineet. I'd definitely pull this in, once I'm back from the vacation. I need to test it too.

Btw, just want to let you know Gazebo 11 is coming to EoL soon. Probably, you might want to take a look at the migration that we are doing for the same. Check out the fortress migration branch.

if you are looking to do some open source contribution in that regards, you can always contact me.