Precursor ion purity calculations for mzXML and other file formats

[Tomnl]

The msPurity precursor ion purity calculation currently parse mzML files to automatically extract the isolation windows.

This is an issue if a user is using either an mzXML file or other file format (see #50)

Although mzML is the standard file format to use for proteomics and metabolomics, for backward compatibility it might be useful to have the functionality to extraction the isolation windows for other file formats such as mzXML.