Major bugfixes for multi-arm setups - Resolves ActionServer and gripper issues

Please see commit message 75ba8af for the full list of changes made. The short version is that this commit (and the next 6f3eba9) fixes two major issues: 

1) Multiple action servers. Currently, trying to initialize more than one arm only allows the latest ActionServer to stay running. This has to do with the way the YouBotArmConfigurations are handled and is fixed in this commit.

2) Gripper position reporting. The current method of throttling gripper position messages inadvertently makes it impossible for more than one gripper to report its position (ever). This code adds multi-arm support while maintaining the throttling as desired.

One minor fix:
Arm topics are hardcoded to be "arm_1", "arm_2", etc., despite having custom names in the launch file (from an earlier pull request I had made nearly two years back that ended up being piecewise integrated to the main codebase). I changed the line at YouBotOODLWrapper.cpp:136 to use the given name rather than the hardcoded value as was originally intended.

Please feel free to contact me with any questions: bradley.h.hayes@yale.edu