[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
minor fix for lib arch check (removed invalid space in regex)
Merge pull request #988 from timosachsenberg/feature/EmpiricalFormula_refactoring

[FIX,FEATURE,API] simplified EmpiricalFormula