Initial messages for review.

[mikepurvis]

Please review, @rgariepy, @ibaranov-cp

Additional MCU msgs/srvs (to be documented on the jackal_firmware ROS wiki page):

nmea_msgs/Sentence -> raw strings to and from globaltop module
sensor_msgs/IMU -> raw data from accelerometer and gyro
geometry_msgs/Vector3Stamped -> raw magnetometer data
std_srvs/Empty -> enable_motors, stop_motors services
std_msgs/Bool (?) -> wifi connected status

MCU parameters to be reviewed separately.

[mikepurvis]

Provide an indication when the system has switched from user control to current-limit control? This is a very real thing which will have to be managed; it'd be great for users to have some visibility into it. It's not per-Drive feedback, though, since the current limit is shared between the two sides (it's the headroom after the PC and user supplies are accounted for).

[rgariepy]

Placeholder for MCU firmware comments:
sensor_msgs/IMU isn't meant for truly raw data. Will need to determine where is best to scale to m/s^2, etc

[mikepurvis]

re: sensor_msgs/IMU

The intent is that the MCU will publish a data message suitable for consumption by the filter, using standard REP-103 units.

So, scaling will occur on the MCU, controlled by rosparams.