'intensity weighted mean' averaging added to ProteinQuantifier, solves #830

This pull request adds the option of 'intensity weighted mean' to the ProteinQuantifier tool.
@cbielow "sum" and "weighted_mean" lead to quite different abundances.

IMO, spectral intensity and therefore peptide intensity reflect the amount of material in the sample. Say we have a protein A with peptides x and y. 4 molecules of x and 6 molecules of y were detected during peptide quantification. Then 5 (or 5.2) rather than 10 molecules of protein A were in the sample. The sum would be my last choice.

I gonna test the four averaging methods on a fixed ratio protein sample after the merge.