Add functionality for calculating the SO2 metric values

[stephhazlitt]

Functionality for assessing data completeness criteria and calculating the SO₂ metrics should follow the methodologies, procedures, and requirements found here.

• The SO₂ metrics include (i) the arithmetic average over a single calendar year of all 1-hour average SO₂ concentrations and (ii) the 3-year average of the annual 99th percentile of the SO₂ daily maximum 1-hour average concentrations.
• Raw, verified air quality data is available in the BC Data Catalogue under the Open Government License-BC.