hubers: Add function to clean clusters.

Add function to clean clusters by filtering track association and time.
Clusters around the maximum cluster are merged into it depending on the
specified merge distance. Neccesar due to artificial cluster splitting
mainly in MC.
hubers: Add function to get maximum cluster.

Add function to get maximum cluster from std::vectors.
hubers: Add function to generate a form factor cut.

Add function to generate form factor correction. Currently hardcoded
for Nickel. Could go to cuts. SOme cleaning up of the code.
first attempt of implementing bgt track cut
Add pi0 to the constants.

Add neutralPionMass() to constants.
start implementing closest pi0 function