Add frequency correction support. The drift factor is currently

Add frequency correction support. The drift factor is currently
determined via linear regression of the last 8 offset corrections.
It is bound to 128 ppm in one step to avoid extreme drifts when
the drift calculation is incorrect for some reason. With the
default values this can result in a maximum wrong drift of
8 * 60 * 8 * 128e-6 s ~= 0.5s (samples per update, update interval,
samples to trigger drift correction, max. drift error) until the
next iteration is done.