digital or analogue read or write of a touch pin breaks other touch pin

[martinwork]

Related: microsoft/pxt-microbit#6072

With the sample below, P2 touch works until button A is pressed/

I suspect this line needs a "-1". I'll test and make a PR.
https://github.com/lancaster-university/codal-core/blob/master/source/drivers/TouchSensor.cpp#L111

#include "MicroBit.h"

MicroBit uBit;

void onButtonA(MicroBitEvent e)
{ 
  uBit.io.P0.setDigitalValue(0);
}

void forever()
{
    while (true)
    {
      if ( uBit.io.P0.isTouched())
        uBit.display.print('0');
      else if ( uBit.io.P1.isTouched())
        uBit.display.print('1');
      else if ( uBit.io.P2.isTouched())
        uBit.display.print('2');
      else
        uBit.display.clear();

      uBit.sleep(100);
    }
}

int main() {
    uBit.init();

    uBit.messageBus.listen( MICROBIT_ID_BUTTON_A,  MICROBIT_BUTTON_EVT_CLICK, onButtonA);

    uBit.io.P0.isTouched( codal::TouchMode::Capacitative);
    uBit.io.P1.isTouched( codal::TouchMode::Capacitative);
    uBit.io.P2.isTouched( codal::TouchMode::Capacitative);

    create_fiber( forever);

    release_fiber();
    return 0;
}

[martinwork]

PR lancaster-university/codal-core#179