[FEATURE] change file format of cached files

- write spectra and chromatogram number at the end of the file
- allows to start writing spectra without knowledge of the number of
  data to write
- [DOC] fix output
[DOC] clearer documentation
[API] simplify API of MSDataCachedConsumer

- it is now longer necessary to know the number of spectra and
  chromatograms beforehand
Merge pull request #816 from hroest/feature/CachedMzMLFormatChange

[FEATURE,FORMAT] cached mz ml format change