[FIX] mzXML fix (filterLine, basePeakMz)

a small fix that allows to read and write the filterLine attribute in mzXML files
true, we could. however storing a large amount of optional attributes is not very useful unless there really is a software that relies on them (as in the case of `basePeakMz`). but you are right, `basePeakIntensity` we could add