Add indices to FollowJointTrajectory to store trajectory index being executed

[Levi-Armstrong]

This is to address issue #65

Open Question: Should it be storing an index for each joint or just a single index for all joints? I believe it is the former.

[destogl]

Open Question: Should it be storing an index for each joint or just a single index for all joints? I believe it is the former.

I think it is sufficient to have one index for all joints because the controllers usually take cases about individual points (all joints) they are passing through.

What is the exact purpose of these indices? Do we need a more descriptive field name?

[Levi-Armstrong]

Currently we use this for dynamic planning. So based on the environment if we are approaching a collision object we use this index to then look forward some time in the trajectory and replan a new segment and splice it in on the controller side. Another use case is if the tolerance violation is triggered during execution this index is useful for restarting the trajectory.

[bmagyar]

@Levi-Armstrong is this still required? Could you please document the new field please? The current name gives 0 clues to the user as to what they should be expecting those indices to correspond to and why.

[Levi-Armstrong]

@bmagyar

is this still required?

I would say yes. This is the index from the provided trajectory that the controller is executing toward. Though I don't think this needs to be a vector; do you agree?

Could you please document the new field please?

I will update the documentation.