[FIX] Export LTAS using PBS Jobs

[Gautzilla]

This PR resolves #338

It adds the deserialize_spectro_or_ltas_dataset() util function that deserializes either a LTASDataset (if the nb_time_bins key is present in the .json file) or a SpectroDataset.

Using this function in the export_analysis main script (which PBS jobs point towards) ensures LTASDatasets are correctly deserialized as LTASDatasets (and not as SpectroDatasets).