single layer counts can now be updated on-the-fly

[ricardogsilva]

This PR is connected to #13

Notable changes:

• Added integration with django-jsonfield-compat. This means that we can now use django's native JSONField, even if geonode is still using django-jsonfield. This means that we can take advantage of the features provided by django's JSONField, such as queryset lookups.

• Added the gfdrr_det.utils.calculate_taxonomic_counts function. It is able to get counts from an exposure layer and takes an optional bbox_ewkt parameter. When a bbox is supplied, counts are gotten from the database by leveraging PostGIS' ST_Intersection function.

• The new function that calculates single layer taxonomy counts has been wired to both the ingestexposures management command and to the views.ExposureLayerViewSet. In the ingestion phase counts are calculated for the whole layer. At runtime, the client can supply the bbox={x0},{y0},{x1},{y1} query parameter and the counts are computed on-the-fly.

The following are two examples of requests and responses that show the pre-calculated values and the calculation on-the-fly (both requests are asking for layer 'uga_v10_buildings_136'):

/gfdrr_det/api/v1/exposure/327

{
  "id": 327,
  "type": "Feature",
  "geometry": {
    "type": "Polygon",
    "coordinates": [
      [
        [
          29.5564174652,
          -1.4970809221
        ],
        [
          29.5564174652,
          4.2136311531
        ],
        [
          35.027256012,
          4.2136311531
        ],
        [
          35.027256012,
          -1.4970809221
        ],
        [
          29.5564174652,
          -1.4970809221
        ]
      ]
    ]
  },
  "properties": {
    "url": "http://10.0.1.95:8000/gfdrr_det/api/v1/exposures/327/?format=json",
    "title": "Uga v10 buildings 136",
    "name": "uga_v10_buildings_136",
    "description": "Uganda exposure v10 by ImageCat",
    "category": "buildings",
    "aggregation_type": "aggregated",
    "wms_url": "http://10.0.1.95:8080/geoserver/hev-e/wms",
    "counts": {
      "data": {
        "category": "buildings",
        "taxonomic_categories": {
          "counts": {
            "construction_material": {
              "unknown": 110617,
              "masonry": 526429,
              "wood": 134913,
              "concrete": 18283
            },
            "occupancy": {
              "unknown": 790242
            },
            "construction_date": {
              "unknown": 790242
            }
          }
        },
        "taxonomy_source": "GEM taxonomy",
        "area_type": "aggregated"
      },
      "name": "Number of assets"
    }
  }
}

# the following bbox parameter defines a rectangle over northwestern Uganda
/gfdrr_det/api/v1/exposure/327/?bbox=29.796695,2.354701,32.857507,3.853763

{
  "id": 327,
  "type": "Feature",
  "geometry": {
    "type": "Polygon",
    "coordinates": [
      [
        [
          29.5564174652,
          -1.4970809221
        ],
        [
          29.5564174652,
          4.2136311531
        ],
        [
          35.027256012,
          4.2136311531
        ],
        [
          35.027256012,
          -1.4970809221
        ],
        [
          29.5564174652,
          -1.4970809221
        ]
      ]
    ]
  },
  "properties": {
    "url": "http://10.0.1.95:8000/gfdrr_det/api/v1/exposures/327/?format=json",
    "title": "Uga v10 buildings 136",
    "name": "uga_v10_buildings_136",
    "description": "Uganda exposure v10 by ImageCat",
    "category": "buildings",
    "aggregation_type": "aggregated",
    "wms_url": "http://10.0.1.95:8080/geoserver/hev-e/wms",
    "counts": {
      "data": {
        "construction_material": {
          "concrete": 1505,
          "masonry": 52685,
          "wood": 13336,
          "unknown": 11514
        },
        "occupancy": {
          "unknown": 79040
        },
        "construction_date": {
          "unknown": 79040
        }
      },
      "name": "Number of assets"
    }
  }
}