[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
[FIX] fix mzMLFile::transform

- there is no more need to pre-read the data to get the number of
  spectra/chromatograms since the interface does not require this
  explicitly any more
[FIX] adapt mzXMLFile::transform