implement pie charts

[jameshadfield]

Pie charts are to be used to implement categorical variables on the map, with area used to represent the number of cases in the deme. @trvrb do you want this to replace chloropleths, as currently implemented, as well?

[jotasolano]

@trvrb @jameshadfield is the idea to do pie charts for v1 and then test them with real data? (as per https://seattle-flu-study.slack.com/archives/CFAEU7MQV/p1555604075011900?thread_ts=1555410693.016500&cid=CFAEU7MQV). If so, are pie charts something we want for May?