[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 in FileConverter
its rather the case that it is not possible with our data model. To convert spectra to chromatogram, you need to iterate through all spectra and collect the data to write to a chromatogram. this would require to keep all data in memory until the last spectrum, so this does not work with the low memory option