fixed PID loop rate

[robz]

right now, PID runs whenever the lm4f receives data from the computer. this is assumed to be 10 Hz, but it would be better if the lm4f guaranteed a fixed rate using interrupts

[weng-frank]

Also, 10Hz seems slow. I'd recommend something on the order of 100Hz.

[kegilbert]

When the PID loop rate was dictated by the communication speed, we couldn't go much faster than 20Hz without getting artifacts in the comm data. Now that Robby simplified the communication, it should be pretty trivial (and a better design) to up the PID rate and have it run independently of receiving data.