Test and change hall sensor reading in Calibration::isInField

[V3lop5]

Previously the magnetic field measured with analogRead. Due to poor performance of over 100 μs this must be changed to digitalRead. This change is not tested so far.

clock/src/Calibration.cpp

Line 134 in f4e1b09

return digitalRead(this->hall_pin) == LOW;

If digitalRead works fine, we should change it to FastGPIO::Pin<THE_HALL_PIN>::isInputLow().
This change will increase the performance from 5.1 μs to 0.125 μs.

After that change the clock communication should be more stable, and we can increase the frequency even more.