Refactor request parsing about gaia

[mbertrand]

@kotfic I replaced the GaiaRequestParser with a slightly modified version of your custom_json_deserialize JSON function in the postgis branch, and modified the request JSON accordingly. Example:

{
  "_type": "gaia.geo.processes_vector.WithinProcess",
  "inputs": [
      {
          "_type": "gaia.inputs.VectorFileIO",
          "name": "first",
          "uri": "{basepath}/iraq_hospitals.geojson"
      },
      {
          "_type": "gaia.inputs.ProcessIO",
          "process": {
              "_type": "gaia.geo.processes_vector.BufferProcess",
              "buffer_size": 1000,
              "name": "buffer",
              "inputs": [
                  {
                      "_type": "gaia.inputs.VectorFileIO",
                      "uri": "{basepath}/iraq_roads.geojson",
                      "filters": [
                          ["type","=","motorway"]
                      ]
                  }
              ]
          }
      }
  ]
}

[mbertrand]

#58

[kotfic]

@mbertrand sorry for the turn around time on review. Looked at deserialize function in the postgis branch and it looks good to me. Once that's in i think we can close this.