Change to use ADC timer for higher resolution input.  This also uses the previously unused input volume value for input attenuation.
Fix documentation for input_attentuation().
Simplify the ADC interrupt handler.  Move the majority of the work in to a bottom-half handler.  Add DCD logic and remove the need to rely on volume squelch.  Had to reduce the serial input buffer a bit.  The bottom-half handling for TX and RX should be cleaned up a bit.  Maybe use the now unused timer for this?
Update copyright.
Merge ADC improvements and DCD feature.