[FEATURE] improved purity computation for IsobaricAnalyzer

This pull request adds a new approach for the purity computation based on Savitski et al. 2011
is this meant as "fraction of peaks" or is "fraction of TIC" ? the latter would make more sense to me since if there are 20 peaks in the one window but one precursor peak accounts for 99% of the TIC I am not so worried about the other 19, right?