improve export -->import consistency

[florian-huber]

We should check if we can export data to a file and import it again without larger losses.
Currently, of I export spectra to mgf files and load them again, parent_mass entries will become a string.
This is not ideal, because it inhibits things like:

• save partly-processed data as mgf.
• start again from this data and run further cleaning steps

This will often fail, for instance if I first run default cleaning steps and later want to run REQUIRE_COMPLETE_ANNOTAION, this will fail in require_parent_mass_match_smiles, where the line mass_difference = parent_mass - smiles_mass won't work anymore.

[florian-huber]

The question here is:

• should such type handling be part of the importing functions
• or should it be in Metadata (where we now already have this for a few fields?