Refactor history of preprocessed data

[SarahAlidoost]

In the notebooks (after merging #38), an attrs dict is defined to hold some information about data like description and history. For history, we list the file names of raw data manually, for example, see below. It would be good if we refactor this part.

attrs={"description": f"Contains modified {INSTITUTION_ID} {METHOD} data used for Atlas in EUCP project.",
           "history": f"original {INSTITUTION_ID} {METHOD} data files CNRM_atlas_tas_CMIP6_histssp585_JJA_latlon.nc, CNRM_atlas_tas_CMIP6_histssp585_DJF.nc" }

[SarahAlidoost]

Also, we can add a reference and the full name of INSTITUTION_ID and METHOD in "description".