More flexible AQ Stats

[jack-davison]

Feature request

Currently, aqStats() is a fairly prescriptive function that spits out fixed and limited air quality statistics that are mainly useful for UK data.

Recent work I've done in the Middle East has required totally different summary stats that openair can calculate, but you need to go round the houses to do so.

I wonder - could we have a more flexible function that allows users to define their own aq limits/stats?

[mooibroekd]

I would really like this. While the aqStats() function follows the EU legalization to some extent, there are some other interesting statistics (like SOMO35 for example) that are not easily implemented.

I have an expanded version of aqStats() that has some extra calculations.