Add normalization information

[AndrewEdmonds11]

For analysis, we will want to normalize our histograms to the number of captured muons. We should include some way to easily include this information so it's available to TrkAna tree analyzers. I'm not sure whether this would be a new branch, or a separate histogram in the same file (like genCountLogger). However, I do think this information should come from Offline. By which I mean, the code to calculate / extract the number of stopped muons is done there, and in TrkAna we just have code to grab the answer.