[FEATURE] add a low memory FileConverter option

- add a low memory option for mzXML/mzML -> mzML conversion
- add an indexed mzML option to produce indexed mzML
[STYLE] style fix
Merge pull request #969 from OpenMS/feature/LowMemFileConversion

[FEATURE] add a low memory FileConverter option
[BUILD] fixed small error in install targets
[FIX] simplified EmpiricalFormula
[INTERNAL] further cleanup and speed-improvements of EmpiricalFormula
[INTERNAL] made typedef more OpenMS like
[FIX] replaced string conversion by appropriate test