change format of ionization/scintillation collected signal

[dguff]

working on this issue on the same dev branch as #19

[dguff]

Large commit in afe96f3e10ae83f188684b922e7a44c085866c30

• Change of the data format: hits (both photons' and ionization's) are now stored as a map with key given by the time in custom clock units (short int) and with value given by the nr of hits reaching the readout at that time (also short int). This significantly reduces the size of the output file.
• We introduced optional BacktrackerRecords to retain some of the information lost with this new data format. Using command macros one can activate the backtracker for different detector subsystem (charge, vuv sipm, supercell). For instance, for each channel one can record che nr of hits produced by a given track (track ID) at each time.

Need of some testing before merging, but I should be close.