Unified column header in QFeatures #10

When reading the data we should use unified column headers for downstream processing.
Current header in MAF are (plus potential mapping)
database_identifier
chemical_formula
smiles => smiles
inchi => inchi
metabolite_identification
mass_to_charge => mz
fragmentation
modifications
charge
retention_time => rtime (in seconds)
taxid => taxid
species => species
database
database_version
reliability => msi
uri
search_engine
search_engine_score
smallmolecule_abundance_sub
smallmolecule_abundance_stdev_sub
smallmolecule_abundance_std_error_sub