Msconvert needs to specifically extract just MS2
MS2 filtering comes first.
PeakList cannot take null peaks array.
Explicitly define dependency on the mzml plugin.
Removing mentions of .mgf, adding mzML
mzXML convertor must convert the peaks as well
peaklist readers need to know whether to read peaks from the construction time

Added extra tests that actually check the conversion.
Testing the spectrum info joiner, currently just with the .mgf-style titles.