[DiffDriveController] Rename "velocity" to "command" or even better "velocity_command"

[destogl]

Current name “velocity” is confusing and ambiguous.

I am actually proposing to rename the "velocity" to "command" or something like this.

Originally posted by @destogl in #260 (comment)

[bailaC]

@destogl this issue can be closed I guess...found velocity_command in current code.