Backward fit and chi2

[kmcdermo]

As we have discussed at length offline and in PR #186 , it was noticed that the bkfit produces some wild chi2 that have to be truncated.

This is an indication that our approach to the bkfit may not be entirely the optimal (simply starting with the last layer x 100 in uncertainty) or we ought to be considering outlier rejection in order to keep the chi2 sane.

This actually affects performance on the CMSSW MTV side, as these candidates are dropped and results in some loss in efficiency (and only gets worse with PU).

Again, as with Issue #188, it might best to wait until we know whether or not we will use the 5x5 representation and if so when it goes in, to look into this.