feat:dashboard aggregations

[NicoPL]

DONE:

• deal with data series (eg. relative_abundance)
• workaround to split data that is merged into rainfall+wind and couv_slope+cover
• configuration dictionary where is written what function to apply to each field, in new constants.py file
• write the right formula to compute standard errors within the MRP function
• integrate with new datasets
• get the year in external data
• use 'project' as a new root level

STILL IN PROGRESS:

• fix: formula for fields with dicts (first: mean in stratum, then weighted mean of stratum means)
• feat: write other aggregation functions (SRS, SS, etc.)
• feat: write other aggregation functions (total and proportion)
• feat: make generic using settings for stratums detailed in "xxx_samp.parquet" files
• feat: support inventaire_bio and enquete datasets
• refactor: reorganize files
  • keep in stats.py only the functions to compute aggregation
  • put in models.py the objects dealing with data and calling aggregation functions
• fix: remove 'for' field to 'loc2' once updated in style.json