DigiEventSparse reconstruction has no physical sense

[chiaratomaiuolo]

By now, the sparse readout strategy is needed only for creating DigiEventSparse digi events, which reconstruction is not even well defined.

In clustering.py, for the class DigiInputFileSparse, we only take all triggered pixels, zero suppress them and then the result clustered, without an effective physical meaning, so, recon files with readoutmode == SPARSE does not have to be used for Physics analysis.

In the future proper criteria for DigiEventSparse should be thought and implemented if needed.