[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
[FEATURE] added option to PPHiRes to specify which ms levels should be picked
[TEST] added test for selecting spec levels when picking
[PARAM,TOPP] removed ms1_only parameter and replaced with ms_levels
[DOC] documented param change in changelog
[NOP] fixed cppcheck performance warnings
[RESOURCE] site R added to unimod #199 and #510
Merge pull request #977 from aiche/feature/pphires-customizable-mslevels

[FEATURE] replaced pphires ms1_only flag with ms_levels parameter
[FEATURE] add library check on Windows for correct lib arch (avoid nasty linker errors)
Merge pull request #983 from cbielow/feature/lib-arch-check

[FEATURE,BUILD] add library check on Windows for correct lib arch
Merge pull request #984 from lars20070/fix/methylSILAC

[NOP] add heavy methyl SILAC labels to unimod.xml
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