Add localization

[ricardogsilva]

This PR adds localization to the backend, making it possible to translate the system to both Italian and English.

The backend deals with two types of translatable string:

• Static strings, which relate to the backend business logic.

  These are things such as the translation of smoothing method names (e.g. EN: centered 5-year moving average IT: media mobile centrata a 5 anni). For these, this PR introduces babel as an additional dependency and provides CLI commands to deal with the usual strategy of working with message catalogs. The typical workflow for translating these is thus to run:

  arpav-ppcv translations extract
  arpav-ppcv translations update
  # now work on translating the files in the /translations dir
  arpav-ppcv translations compile

  Included in this PR is a first pass of translating these static strings to Italian, done with google translate.

• Dynamic strings, which relate to the configuration that is created via the admin.

  These are things such as the display name and description of coverage configurations, configuration parameters, etc. This PR enhances the relevant database tables with columns suitable for storing translation of these dynamic properties. Here is a screenshot of the admin editing screen for a configuration parameter:

  

  Note that this PR does not include bootstrap values for providing translated names of all stock configurations - This shall be done in a subsequent PR.

This PR includes also the integration of these translated values in with the time series API endpoints, thus providing information to the frontend so that it may localize generated charts. This a sample request:

http://webapp:5001/api/v2/coverages/time-series/tas_seasonal_absolute_model_ensemble-rcp26-DJF
  coords==POINT(11.5469 44.9524)
  datetime==../..
  include_coverage_data==true
  include_observation_data==true
  coverage_data_smoothing==NO_SMOOTHING
  coverage_data_smoothing==MOVING_AVERAGE_11_YEARS
  include_coverage_uncertainty==true
  include_coverage_related_data==true

Would return a response that now includes extensive details about the configuration and translations:

[
  {
    "name": "tas_seasonal_absolute_model_ensemble-rcp26-DJF",
    "values": [...],
    "info": {
      "processing_method": "NO_SMOOTHING",
      "coverage_identifier": "tas_seasonal_absolute_model_ensemble-rcp26-DJF",
      "coverage_configuration": "tas_seasonal_absolute_model_ensemble",
      "scenario": "rcp26",
      "year_period": "DJF"
    },

    "translations": {
      "parameter_names": {
        "series_name": {
          "en": "series name",
          "it": "Nome della serie"
        },
        "processing_method": {
          "en": "processing method", 
          "it": "Metodo di elaborazione"
        },
        "coverage_identifier": {
          "en": "coverage identifier", 
          "it": "Identificatore di copertura"
        },
        "coverage_configuration": {
          "en": "coverage configuration", 
          "it": "Configurazione della copertura"
        },
        "scenario": {
          "en": "Scenario", 
          "it": "Scenario"
        },
        "year_period": {
          "en": "Year period", 
          "it": "Periodo dell'anno"
        }
      },
     "parameter_values": {
        "series_name": {
          "en": "TAS seasonal absolute model ensemble", 
          "it": "TAS valore assoluto di stagione media ensemble"
        },
        "processing_method": {
          "en": "no processing", 
          "it": "nessuna elaborazione"
        },
        "coverage_identifier": {
          "en": "tas_seasonal_absolute_model_ensemble-rcp26-DJF", 
          "it": "tas_seasonal_absolute_model_ensemble-rcp26-DJF"
        },
        "coverage_configuration": {
          "en": "TAS seasonal absolute model ensemble",
          "it": "TAS valore assoluto di stagione media ensemble"
        },
        "scenario": {
          "en": "RCP2.6", 
          "it": "RCP2.6"
        },
        "year_period": {
          "en": "Winter", 
          "it": "Inverno"
        }
      }
    }
  },
  ...
]

Notable things about the response snippet pasted above:

• the info key contains all configuration parameters of the coverage. This is added in the spirit of #162, with the idea being that the frontend shall be able to better parse the response information with this added information

• the translations key has two children: parameter_names and parameter_values. These provide appropriate translation for both the names and values of the fields present in the info key.

Also included in this PR (admittedly somewhat unrelated) is a refactor of the way time series data is fetched from the remote THREDDS server. The proposed implementation leverages anyio and httpx.AsyncClient in order to interact with THREDD's NCSS service in an async fashion. As commented in #163, this was done with the hopes of improving performance of the system.

---

• fixes #153
• fixes #106
• fixes #163