Create "UKCP18_variables.json" vocabulary file

[agstephens]

Base the file on content here:

https://docs.google.com/spreadsheets/d/1Ij3R3skvYhKnMSqXB6KHaxH0BSST5R0DI8zp2Qi82vw/edit#gid=762056270

Use structure along the lines of:

{
    "variables": {
        "annual_airfrost_climatology": {
            "bounds_duration": "",
            "bounds_start_offset": "",
            "bounds_type": "",
            "cell_methods": "",
            "historic_name": "daf",
            "long_name": "",
            "scalar_coordinate_variables": "",
            "short_name": "annual_airfrost_climatology",
            "standard_name": "",
            "temporal_resolution": "annual",
            "time_point_offset": "",
            "units": "",
            "variable_name": "_airfrost"
        },
  },
...lots more variables...
    "version_metadata": {
        "author": "Ag Stephens <ag.stephens@stfc.ac.uk>",
        "creation_date": "Tue Jun 06 07:45:06 2017 -0100",
        "institution_id": "STFC",
        "previous_commit": ""
    }
}

Things to be aware of:

• Where multiple rows have the same variable ID we should have 1 variable entry but add an attribute with a list of the possible:
  • "frequencies"
  • "time averages"
  • "strands"

Useful references:

1. CMIP6 MIP Tables: https://github.com/PCMDI/cmip6-cmor-tables/blob/master/Tables/CMIP6_3hr.json
2. UKCP18 obs variables: https://github.com/ukcp-data/UKCP18_CVs/blob/master/UKCP18_obs_variables.json

[agstephens]

Re-assigning to @agstephens because I have some code to generate these.

[agstephens]

Details and code here that explain reading a Google doc directly into python:

https://github.com/cedadev/ukcp18-data/tree/master/code/gdoc_lib

[agstephens]

Needs updating after recent review of the Variables Google spreadsheet.

[agstephens]

Done.