AP_InertialSensor: always sample at 200Hz in MPU6000

this changes the sampling to 200Hz regardless of requested rate, and
it is downsampled inside num_samples_available() using a shift. This
gives better noise resistance in ArduPlane.

This patch also makes it possible to update the filter frequency while
running, which is very useful for bench testing with a vibration
source
AP_InertialSensor: removed unused new_data_available() and temperature() APIs
AP_InertialSensor: added filter frequency support to PX4 driver