Add GeoJSON hazard curve parser and writer

[Lars Butler]

We have NRML/XML-based parsers and writers; it would be useful to support GeoJSON as well.

Here is what the format could look like:

{'features': [{'geometry': {'coordinates': [-116.0, 41.0], 'type': 'Point'},
               'properties': {'poEs': [0.98728, 0.98266, 0.94957]},
               'type': 'Feature'},
              {'geometry': {'coordinates': [-116.0, 42.0], 'type': 'Point'},
               'properties': {'poEs': [0.999, 0.5, 0.0001]},
               'type': 'Feature'}],
 'oqmetadata': {'statistics': 'quantile',
                'quantileValue': '0.15',
                'IMT': 'SA',
                'saPeriod': '0.025',
                'saDamping': '5.0',
                'investigationTime': '50.0',
                'poE': '0.1',
                'IMLs': [0.005, 0.007, 0.0098]},
 'oqtype': 'HazardCurve',
 'oqnrmlversion': '0.4',
 'type': 'FeatureCollection'}

[Lars Butler (lars-butler)]

Pull request: https://github.com/gem/oq-nrmllib/pull/108