Calibration update

[chrisib]

Several changes related to calibration:

1. Remove calibration & diagnostic tools from their old locations, move them both into a new firmware/tools namespace
2. Rewrite calibration to support low-accuracy mode with 5V input as well as the old low-10V and high-10V modes (there seem to be relatively few EuroRack modules that generate 10V offset voltages, but 5V is pretty widely available through a variety of modules. This new mode should simplify fast (re-)calibration using only modules in the same case.
3. Change output calibration to calibrate each output channel instead of just CV1. This should make e.g. quantizers more usable on other channels. europi.py is modified to work with both the new & old calibrations, so anyone who's calibrated using only the CV1-only method will still have usable calibration.
4. Add new Thermometer class to europi.py to act as a wrapper for the temperature sensor on pin4. Add exception-handling needed for the Pico 2 to prevent crashes when trying to read the temperature sensor.