Fix mvd clusterfinder

Change the code such that the cluster finder also works event based.
Loop over all frames such that all data is written to the output after the
Exec function.
Refactor code.
Move code for cleanup in extra function.
Since TClonesArray->AbsorbAt() removes entries from the array looping from
front to back doesn't work correctly such loop from back to front.
Mark digis below the threshold as used such that they are removed from the
input array during cleanup.