Fix the "fraction" definition

[drinkingkazu]

This addresses the issue #137

[soleti]

I think this can safely be merged, but maybe one extra person should independently verify this. @krwood?
@drinkingkazu, just to double check, you want to add also the 2x2 notebook?

[krwood]

@YifanC is going to handle the testing and approving of this PR. IMO one thing that might be nice to see is summed fractional contributions from a sufficiently complex event for (1) before this PR with MAX_TRACKS_PER_PIXEL set high vs. (2) before this PR with MAX_TRACKS_PER_PIXEL set low vs. (3) after this PR with MAX_TRACKS_PER_PIXEL set high vs. (2) after this PR with MAX_TRACKS_PER_PIXEL set low.

I do agree with @drinkingkazu and @soleti that this seems like a nice improvement. We may even want to put in an error or warning message that is triggered when the summed fractional contributions fall short of 100%?

[krwood]

Pull Request #140 is very similar to this one, but it also implements a fractional contribution normalization that is in essence q_sum but without the noise superimposed on the signal. I suggest we merge #140 and close this one, but please shout if you disagree @drinkingkazu.