implement log file writer for simulation tasks

[markusachtelik]

• @ffurrer how do we get the data from gazebo? cpp / python?
• format: see logging format / method #19
• possibility to enable / disable logging

Data to log:

• ground truth
  • position
  • linear_velocity
  • attitude quaternion
  • angular velocity
  • in_collision ?
• simulated sensor data
  • IMU
  • generic pose sensors
• motor_speeds
• control commands on all levels according to our message definitions Control* in https://github.com/ethz-asl/mav_comm/tree/master/mav_msgs/msg.
• services / signals that we emit to the contestants
• parameters. We only need to be careful not to expose internal parameters here that the contestants are not supposed to know.

[ffurrer]

Done in #23