Upgrade the code timing.cc to use THnSparseI instead of TH3I

Description

At the moment, the code timing.cc used to extract time calibrations for different detectors makes heavy use of TH3I histograms; in these histograms, the y-axis is typically used to store, as a label, the individual detector ID. Later, this axis is projected to study individual detector responses.

The use of a common TH3I histogram is indeed a convenient solution, but it requires lot of memory.

A simpler way can be the use of the new THnSparse class - since only a small number of histograms will be filled.