this exposes the product ID to the IMU Layer

this returns the expected max drift rate for the particular type of
gyro being used

This reverts commit dafeac01d8f14162600cf821404ba4072dcae14d.

ADC will be disabled on APM2

enable_channel() must be used before the Ch6() method is called to
enable gathering of data on the required channels

this abstracts the way of getting inertial sensor (gyro and
accelerometer) data for the APM1 and purple hardware. The Oilpan code
is based closely on the old APM1 code