[NOP] coding style improvements
[FEATURE,PARAM,DOC] added option to treat isoleucine/leucine as equivalent ('-IL_equivalent'), updated doc; resolves #872
[TEST] test for 'PeptideIndexer -IL_equivalent'
Merge pull request #927 from hendrikweisser/PeptideIndexer-isobaric

PeptideIndexer: support for treating isoleucine/leucine as equivalent (resolves #872)