Reword filtering parameters to make distinction of usage for hard-filtering and tiering/prioritizing clear.

[susannasiebert]

E.g.

-b BINDING_THRESHOLD, --binding-threshold BINDING_THRESHOLD
                        Report only epitopes where the mutant allele has ic50
                        binding scores below this value. (default: 500)

This should be reworded to make clear that the hard-filtering is only done for the filtered.tsv file. For the aggregated file this parameter is used for tiering and for determining the Best Peptide.

Also reword other parameters like the TSL and coverage/expression parameters.