[FIX] remove const, allow consumers to change data

- the consumers may wish to change the spectra and chromatograms,
  specifically modify the raw data or remove it completely
[FIX,API] adapt to change in OpenMS API r11592
[FIX,API] adapt to change in OpenMS API r11838
[FIX] allow inheritance
[FEATURE] add MSDataReader implementation for mzXML files