Add support for STRDS export

[mmacata]

This PR initiates export for STRDS. Here, t.rast.export was used which does not support netCDF or similar formats but writes out separate GeoTIFFs and packs them. The output structure of the archive can be seen in the GRASS GIS documentation.

Part of #192

It was tested locally with /api/v1/locations/nc_spm_08/processing_async_export endpoint with modis_lst mapset available. The process chain used was

{
    "list": [
      {
        "id": "t_rast_extract_93486",
        "inputs": [
          {
            "param": "input",
            "value": "LST_Day_monthly@modis_lst"
          },
          {
            "param": "where",
            "value": "start_time >= '2016-05-01' AND start_time <= '2016-05-30'"
          },
          {
            "param": "output",
            "value": "lst_test"
          },
          {
            "param": "expression",
            "value": "1.0 * LST_Day_monthly@modis_lst"
          },
          {
            "param": "basename",
            "value": "lst_test"
          }
        ],
        "module": "t.rast.extract"
      },
      {
        "id": "save_result_333549",
        "module": "exporter",
        "outputs": [
          {
            "export": {
              "format": "GTiff",
              "type": "strds"
            },
            "param": "map",
            "value": "lst_test"
          }
        ]
      }
    ],
    "version": "1"
}

[griembauer]

Thanks a lot, looks good to me, just one small question

[mmacata]

Thanks for the reviews, I will merge now!