Tweak priority
PiOS Sensors: Use field name temperature instead of temp

Using temperature is better since there is no ambiguity with temporary
variables.
PiOS SENSORS: Make pios_sensors.h sufficient for all of pios_sensors.c

This should be the default behavior when possible to facilitate unit testing
of the drivers.
Make some variables and methods static when possible

Based on feedback from https://github.com/PhoenixPilot/PhoenixPilot/pull/163
Formatting improvements and typo fixes

Based on comments in https://github.com/PhoenixPilot/PhoenixPilot/pull/163
Sensors: Set the alarm before the delay when the queues are stalled
PiOS HMC5883: When driver has a problem then add a delay

This stops the task burning 100% CPU when the mag has a bug.  Also only
add data to the queue when it is good.
PiOS L3GD20: Only get the scale one time
PiOS MPU6000: Fix some parentheses that cause errors at some orientations
PiOS MPU6000: Get the scales once and use local variable