Update fitter with angular2D function validated.

A practice for pull request and update fitter by chance.
A method to sync forked repo with upstream is also provided as a reference.

Three commits:
1. Fitter with angular2D function validated.
2. More precise condition to select candidates at GEN level. Skip events with multiple candidates.
3. < regex > is found to be a c++11 feature supported since gcc491. It can be compiled using clang instead of gcc on SLC5/6.

By the following commands, it's allowed to sync your fork with upstream.

#First add upstream repo
git remote -v
git remote add upstream https://github.com/cms-bph/BToKstarMuMu.git
git remote -v
#Fetch upstream changes and merge with master
git fetch upstream
git checkout master
git merge upstream/master
git push --tags

@xshi could you please verify this?