Adds Safe Trajectory Follower implementation

Safe Trajectory Follower implements different approach for controlling
the robot. Rather than calculate the interpolation steps in the driver
and send the small interpolated steps over the network to the URScript
program with 500Hz frequency, the coarser MoveIt trajectory is sent
(with few Hz) and the interpolation steps are calculated by the
URScript.

The algorithm for time progress has also built-in protection against
any delays induced by load on the driver, network or URControl - it
will never "catch-up" dangerously when such delay are introduced,
It will rather pause and wait for the next small interpolation step
instructions and re-start the move slower - never skipping any
interpolated steps.

Those changes make Safe Trajectory Follower much more resilient to
network communication problems and removes any superficial requirements
for the network setup, kernel latency and no-load-requirement for the
driver's PC - making it much more suitable for research, development
and quick iteration loops. It works reliably even over WiFi.

include/ur_modern_driver/tcp_socket.h

@@ -46,6 +46,7 @@ public:
 
   std::string getIP();
 
+  bool read(char *character);
   bool read(uint8_t *buf, size_t buf_len, size_t &read);
   bool write(const uint8_t *buf, size_t buf_len, size_t &written);