Add AFC

[mweyland]

For the system to work properly, the input to the slicer is not allowed to have any DC offset. The slicer does have an alpha-parameter that would allow it to track a moderate DC offset. This parameter is currently set to 0, but it's worth investigating if it could be used for automatic frequency control.

Then again, frequency correction should be done using the Frequency Xlating FIR Filter. The build-in low pass filter would compromise a non-centered signal and the slicer can't do anything about that.