Add endpoints for variables (with descriptions)

[rod-glover]

Return a list of descriptions of variables available for each dataset.

Request

GET /<dataset>/variables

returns

[
    {
        "identifier": String,  // internal identifier, e.g., 'tmax'
        "short_name": String,  // short human-readable name; may contain HTML; e.g., 'T<sub>max<sub>'
        "long_name": String,  // long human-readable name, e.g., 'Maximum Temperature',
        "description": String,  // human-readable full description, e.g., 'Monthly average of daily maximum temperature'
        "units": String  // human-readable units of measure, e.g., 'deg C'
    },
    ...
]

[rod-glover]

Alternatively, returns

{
    "<identifier>": { // internal identifier, e.g., 'tmax'
        "short_name": String,  // short human-readable name; may contain HTML; e.g., 'T<sub>max<sub>'
        "long_name": String,  // long human-readable name, e.g., 'Maximum Temperature',
        "description": String,  // human-readable full description, e.g., 'Monthly average of daily maximum temperature'
        "units": String  // human-readable units of measure, e.g., 'deg C'
    },
    ...
}