[FEATURE] improved purity computation for IsobaricAnalyzer

This pull request adds a new approach for the purity computation based on Savitski et al. 2011
the current scan is handled in the main loop (https://github.com/OpenMS/OpenMS/pull/1055/files#diff-57ec177500a3a499c153552604694cceR441). Another option would be to wrap both into an iterator-like class, but this seemed a bit overkill.