results for population_density, topography_srtm and others return the value entry in the properties dict when called without the agg argument

When calling the population_density service without the agg argument, the result is returned in the properties dict with a key label of null. The correct label (to be consistent with the other services) should be value.

Example -- population_density:

http://127.0.0.1:8000/population_density/?lat=50.909637703990136&lon=6.406305214120635&year=2000

returns

...
   "properties" : {
      "agg_function" : null,
      "many" : false,
      "null" : 4678.08738545859,
      "radius" : null,
      "units" : "1/km^2"
   },
...

Example -- topography_srtm:

http://127.0.0.1:8000/topography_srtm/?lat=50.909637703990136&lon=6.406305214120635&relative=False

returns

...
   "properties" : {
      "agg_function" : null,
      "many" : false,
      "null" : 96,
      "radius" : null,
      "relative" : false,
      "units" : "m"
   },
...

The above also holds for stable_nightlights, ecoregion, climatic_zone, nox_emissions, population_number and landcover; only examples for testing will follow:

http://127.0.0.1:8000/stable_nightlights/?lat=50.909637703990136&lon=6.406305214120635&year=2012
http://127.0.0.1:8000/ecoregion/?lat=50.909637703990136&lon=6.406305214120635&description=False
http://127.0.0.1:8000/climatic_zone/?lat=50.909637703990136&lon=6.406305214120635
http://127.0.0.1:8000/nox_emissions/?lat=50.909637703990136&lon=6.406305214120635&year=2012
http://127.0.0.1:8000/population_number/?lat=50.909637703990136&lon=6.406305214120635&year=2000
http://127.0.0.1:8000/landcover/?lat=50.909637703990136&lon=6.406305214120635&description=False&year=2012