Add event Observation

[lboeman]

I neglected to add an event observation to the last PR. The metadata of the added is as follows:

{'name': 'Weather Station Event Observation',
 'variable': 'event',
 'interval_value_type': 'instantaneous',
 'interval_length': 5.0,
 'interval_label': 'event',
 'site': {'name': 'Weather Station',
  'latitude': 42.19,
  'longitude': -122.7,
  'elevation': 595.0,
  'timezone': 'Etc/GMT+8',
  'site_id': '123e4567-e89b-12d3-a456-426655440001',
  'provider': 'Organization 1',
  'extra_parameters': '{"network_api_abbreviation": "AS","network": "University of Oregon SRML","network_api_id": "94040"}'},
 'uncertainty': 1.0,
 'observation_id': '991d15ce-7f66-11ea-96ae-0242ac150002',
 'provider': 'Organization 1',
 'extra_parameters': ''}
​```

With a valid period between 2019-04-14T0700Z and 2019-04-17T0655Z.