[FEATURE] improved purity computation for IsobaricAnalyzer

This pull request adds a new approach for the purity computation based on Savitski et al. 2011
this formula seems to rely a lot on the sign of the numbers. Therefore it seems critical that 
`followUpScan.getRT() < precursorScan.getRT()`, correct? Otherwise the sign of `late - early` may be reversed by `followUpscan.getRT() - precursorScan.getRT()` correct? I might just feel more safe if there was an `abs` around the two expression that should not be negative...