agc: completely restructuring automatic gain control object

  * re-writing interface
  * completely removing squelch methods (for simplicity)
  * using simplified closed-loop control loop
  * still needs work and testing
  * needs better relationship between bandwidth and response time
agc: adding methods to explicitly set signal level, gain, rssi

  * useful for setting agc's initial gain estimates
  * for low bandwidth, agc can take some time to settle
  * these methods seed the agc's state and significantly reduce the
    amount of time needed to stabilize
agc: adding method to initialize internal gain on input signal

  * user provides buffer of samples
  * agc simply computes RMS level and seeds initial state based on this
    estimate
  * easy way to get the agc object a rough estimate of signal level to
    reduce the initial settling time on start-up
build: removing old agc method prototypes from global internal header
build/agc: removing references of squelch in documentation