Generate forecast menu dynamically

[ricardogsilva]

This PR implements new API endpoints for providing the frontend with an appropriate set of valid combinations for generating a navigation for forecast datasets.

Combinations of parameters are generated like this:

• one entry for each combination of climatological_variable - aggregation_period - measure (e.g. tas-30yr-anomaly)
• for that entry, list all valid combinations of the remaining parameters

This way of combining the existing parameters allows us to correctly represent entries like:

• how the snwdays dataset has both 30yr anomaly and a annual absolute entries and they support different combinations of other parameters
• the HWDI dataset, which is only available in the JJA season
• the fact that TAS annual anomaly does not have a time_window, but TAS 30yr anomaly does have it

for example:

{
      "variable": "tas",
      "aggregation_period": "30yr",
      "measure": "anomaly",
      "other_parameters": {
        "year_period": [
          "MAM",
          "JJA",
          "SON",
          "DJF"
        ],
        "climatological_model": [
          "model_ensemble",
          "ec_earth_cclm_4_8_17",
          "ec_earth_racmo22e",
          "ec_earth_rca4",
          "hadgem2_racmo22e",
          "mpi_esm_lr_remo2009"
        ],
        "scenario": [
          "rcp45",
          "rcp26",
          "rcp85"
        ],
        "time_window": [
          "tw2",
          "tw1"
        ]
      }
    }

The new endpoint is reachable at:

/api/v2/variable-combinations

it returns an object with this structure:

{
  combinations: []
  translations: {}
}

where combinations is a list of objects describing entries as mentioned above and translations is an object with the translation for the various parameters, allowing the construction of a localized navigation in both english and italian

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• fixes provide forecast dataset combinations for the frontend #211