Function to read and extract data from MASCOT Generic Format (.mgf) files
The .mgf format contains minimal metadata (spectrum title, retention
time, precursor mass, precursor intensity and charge) and the
corresponding MS2 spectrum (https://fiehnlab.ucdavis.edu/projects/lipidblast/mgf-files)

INPUT
A character array (1 file) or cell array (multiple files) are optional
input. When the file path is not given as input, files can be browsed for

OUTPUT
The output is a structure containing the following fields:
- scanName: Title given by ProteoWizard (character array)
- precursorMass: Mass of precursor ion (double)
- precursorIntensity: Intensity of precursor ion (double)
- z: Charge of precursor ion (double)
- scanData: MS/MS spectrum (N x 2 cell containing m/z values
and intensities)