Make plotting script flexible to grippers

[buntyke]

Currently the indices for plotting joint angle data are hard coded and can generate wrong plots when the gripper data is included in the file. This needs to be updated in a way that the presence of gripper data is automatically detected and ignored during plotting.

[buntyke]

This flexibility can be programmed by using the code provided in the Baxter SDK example: joint_trajectory_file_playback.py. It is implemented in the _clean_line() function in the program.