PHOS QA class for using in PbPb runs in the PWGPP QA train.

The main objective is to provide a user with an input for pi0 mass,
width and number-of-pi0-per-event trend plots in PbPb
runs. Unfortunately, large combinatorial background prevents from the
direct using of CaloCellQA framework for this, so the event mixing
technique is essential.

This class provides the standard set of 2D-histograms: invariant mass
of two clusters in PHOS vs their pT both for "real" and "mixed" events
for each of the three PHOS modules in centrality bins 0-20% and
20-100% (cen0 and cen1 in the histogram names, resp).  Only clusters
with energy > 0.3 GeV and number of cells >2 are accepted.

"Mixed" histograms are identified by "Mi" in their names.

Requires no configuration.