remove dependency on global `map` object
remove dependency on global `map` object

LSHteam/leaflet-solr-heatmap,mejackreed/leaflet-solr-heatmap,LSHteam/leaflet-solr-heatmap,mejackreed/leaflet-solr-heatmap,LSHteam/leaflet-solr-heatmap,mejackreed/leaflet-solr-heatmap