Implement a DigiCircularReadout variant where the geometrical ordering of the 6 lowest pixels is fixed

[lucabaldini]

While I don't think we will ever implement this is hardware, this looks like a sensible feature to have in that:

1. it will make HDF5 files easier to browse;
2. it will make it easier to apply ML methods to the reconstruction, as the event structures become effectively one-dimensional;
3. it would like make the simulation and reconstruction cleaner and slightly faster.

(In the past I think we had to apply some pre-processing before training NNs on circular events.)

I would go as far as saying that this new readout should be the default for all the simulations.

@augustocattafesta thoughts?

[augustocattafesta]

Totally agree. It would simplify the NN reconstruction interface a lot.