Make sure our robot doesn't tip over while digging by monitoring the pitch angle of our zed pose topic

[BergerKing333]

As discussed at the PDR, we could use the pitch angle of our zed pose topic to determine if we are lowering the digger too low into the ground causing the robot to tilt. This could be a good safety feature to implement if we have time. While digging we would want to slow down (or stop) the lowering of the digger if the robot starts to tip.

This is on hold until #337 is finished first, as we don't have proper control of the dig linear actuators yet.