Add support to export loss maps to GeoJSON

[Lars Butler]

This will add the first piece of support for a new output type: (Geo)JSON.

Possible structure for loss maps as GeoJSON:

{'features': [{'geometry': {'coordinates': [-116.0, 41.0], 'type': 'Point'},
               'properties': {'losses': [{'assetRef': 'asset_1',
                                          'value': '15.23'},
                                         {'assetRef': 'asset_2',
                                          'value': '32.66'}]},
               'type': 'Feature'},
              {'geometry': {'coordinates': [-116.0, 42.0], 'type': 'Point'},
               'properties': {'losses': [{'assetRef': 'asset_3',
                                          'value': '64.23'}]},
               'type': 'Feature'}],
 'oqmetadata': {'gsimTreePath': 'b3_b4',
                'investigationTime': '50.0',
                'lossCategory': 'economic_loss',
                'poE': '0.5',
                'sourceModelTreePath': 'b1_b2',
                'unit': 'EUR'},
 'oqtype': 'LossMap',
 'oqnrmlversion': '0.4',
 'type': 'FeatureCollection'}

NOTE -- The addition of the geojson lossmap writer to nrmllib is covered here: https://bugs.launchpad.net/oq-nrmllib/+bug/1198881

[Lars Butler (lars-butler)]

First pull request: https://github.com/gem/oq-engine/pull/1226

[Lars Butler (lars-butler)]

Second (and probably final) pull request: https://github.com/gem/oq-engine/pull/1227