Implement GNPS loading functionality on DatasetLoader layer

[hechth]

The GNPS loading functionality is implemented for all the parts but not on the layer where it could replace the load_dataset function or _load_metabolomics - we have to decide where to put this functionality and implement the proper class(es).

This would mean replacing the load_gnps.py and metabolomics.py files with the GNPSLoader class.

The GNPSLoader should take all the paths as optional arguments so the user can specify them and if they are not there.

The init function basically has 2 options, either take the root to the folder and figure out the paths yourself or take all individual paths loaded from the config and load the data.

[CunliangGeng]

This is solved in a different way in #196.