Metadata cleanup

[craig-willis]

Preliminary implementation of metadata cleanup/standardization for LemnaTec raw metadata. This PR:

• Adds the Clowder dataset ID for fixed sensor information to the STATIONS dict with get_fixed_datasetid_for_sensor function
• Adds the metadata package with metadata.py and metadata/lemnatec.py. These implement the clean_metadata method for the Lemnatec JSON metadata.
• Preliminary implementation of the point cloud origin calculation

See also: terraref/documentation#99

[craig-willis]

@jterstriep
@max-zilla suggested that I merge this after an earlier meeting. I am making a minor change to the STATIONS data -- adding a Clowder dataset ID for authority/fixed sensor information. I also added a lookup function to be able to get this by station/sensorId, which is required for the cleaning code. Feedback welcome.