Close the loop over the different types of digitizaiton

[lucabaldini]

We have been working on different readout strategies in the more_digi branch, and we do have three DigiEvent structures

• HexagonalReadoutSparse: read all the pixels above the trigger threshold, no matter where they are;
• HexagonalReadoutRectangular: good, old XPOL-like readout with ROT, padding and ROI;
• HexagonalReadoutCircular: highest pixel above the trigger threshold + the 6 neighbors.

We need to do a few things in order for this to be actually used, particularly:

• write all the IO data structures for data persistence;
• write the proper clustering structures to seed the reconstruction;
• adapt the top-level application to select at runtime the specific digitization scheme we want to use;
• adapt the display facilities.