CALO GraphClustering - Add local maxima check on insertion

Some updates regarding the graph clustering algorithm:

• Explicitely check that a seed is a local maxima before inserting it and its neighbours in the graph. Not doing so was causing an increase in the number of clusters of about 10% on every event ("ghost" clusters made out of residual deposits).
• In the fraction calculation for overlap cases, the energy of the overlapping cell was previously taken into account fully for each cluster involved in the overlap, so the total energy of the connected component may have the energy of the overlapping cell accounted twice. Now its energy is only accounted once by fractioning its energy equally between the overlapping clusters before calculating the fractions.
• Update the threshold value for merged pi0 candidates according to new studies.

This MR is ready for testing.