FRI 2.x messages

[mhubii]

Refer #158

• FRI message updates:
  • Cartesian pose support
  • Commanded joint position removal

Requires lbr_fri_msgs to live outside this stack, here: https://github.com/lbr-stack/lbr_fri_msgs

The difficult bit will be to automatically checkout the correct branch at compile time, assuming there is a single .repos file for all FRIs. Alternatively one might just create N .repos files, where N = FRI versions, which should be much cleaner

[mhubii]

messages moved to https://github.com/lbr-stack/lbr_fri_idl in #147 . Not supported yet: cartesian pose