Change test input to a GPIO square wave and move pins around

* Add GPIO square wave module PC.4
* Move ADC to ADC1 channel 15, on PC.5
* Remove DAC1 (no longer used in sample)
* Add bandpass/notch/resonator filter to system
* Pins moved to make it easy to connect square wave to ADC (i.e.
  PC.4 to PC.5)