dynamically setting first dataset selected in leaflet map
commented out raw number imports, relates to #17
leaflet map updates
+ added year selector based on min/max available years for each dataset
+ added categorized dataset selector
+ moved leaflet_community_areas to map
+ enabled map in primary navigation
+ added placeholder choropleth cutoffs for Prenatal care groups
+ added dynamic choropleth cutoffs for each datatset
+ relates to #5