Add functionality for calculating the CAAQS ozone metric value

[ateucher]

Will probably need to generalize a lot of PM_2.5 functions

[stephhazlitt]

Functionality for assessing data completeness criteria and calculating the CAAQS ozone metric should follow the methodologies, procedures, and requirements in the Guidance Document for the Achievement Determination (GDAD) for the Canadian Ambient Air Quality Standards for Fine Particulate Matter and Ozone, pages 13-16.

• The CAAQS ozone metric is the 3-year average of the annual 4th-highest daily maximum 8-hour average concentrations.
• The general approach is written as a script in ozone-caaqs-analysis, lines 1 to 84.
• Raw, verified air quality data is available in the BC Data Catalogue under the Open Government License-BC.
• The results of the 2011-2013 CAAQS ozone metric calculations are also open and should be used to compare and test function outputs.

[ateucher]

NOTE: Due to a change in dplyr 0.5.0 where arrange() now ignores groupings defined using group_by(), the method for selecting the fourth highest daily maximum here (lines 48-54) will no longer work. A different solution will be required.