Fixed issue with digital outputs used as pwm on mxp (#14)

When a digital output object that was tied to an mxp pin had enable pwm called on it
it would pwm on a pin 6 lower
 (although if this wasn't an mxp pin it wouldn't do anything at all.)
Fixed in Digital.cpp by adding 6 if it is an MXP pin in setPWMOutputChannel()
This should fix the CanJaguar test because when digitalOutputs used as PWM
were freed, the PWM generator was set to the number of pins, which meant it
was actually outputing on pin 20.

Change-Id: Ib48db3e6e3bf78659622145969d24011cc231ea6

Updated Hal to include some documentation about swaping mxp pins.

Cannot find NI related documentation as to the pin numbers being wrong though

Change-Id: I71d84431dc9bc7bf22aa29b6633e49723311b5f7

fixed formatting on changed files

Change-Id: I5b7c16cd798132b6b20c1d03f334a95b42d2ee11

hal/lib/athena/DIO.cpp

@@ -126,6 +126,10 @@ void setPWMDutyCycle(void* pwmGenerator, double dutyCycle, int32_t* status) {
 void setPWMOutputChannel(void* pwmGenerator, uint32_t pin, int32_t* status) {
   uint32_t id = (uint32_t)pwmGenerator;
   if (id > 5) return;
+  if (pin >= kNumHeaders) {       // if it is on the MXP
+    pin += kMXPDigitalPWMOffset;  // then to write as a digital PWM pin requires
+                                  // an offset to write on the correct pin
+  }
   digitalSystem->writePWMOutputSelect(id, pin, status);
 }