realtime factor low when conveyor belt in use

[Deanna Hood]

this change is a potential fix (from this PR), but it was reverted in https://bitbucket.org/osrf/ariac/commits/27746d3acc7c02b0fd17fabed23037bb303ccf24 because of https://bitbucket.org/osrf/ariac/issues/15

[Brian Gerkey]

Performance problem seems to result from excessive collision checking following set-velocity on the objects on the belt. Options include:

• set force instead of velocity
• collision bit-mask setting
• discrete instead of continuous motion of objects on belt
• reduce pace of object spawning to have fewer objects on the belt at any one time

[Carlos Agüero]

See pull request #31.

[Carlos Agüero]

Solved in pull request #31