o Support feature.clusterGroup to create multiple separate clusterGroups per map. These operate in their own universe and will not merge.
o Interpret maxClusterRadius==0 to NOT invoke clustering.
o Tidy up doc comments in leaflet_markercluster.api.php
Straight port from D7 7.x-1.x-dev to 8.x-1.x
Needed some changes in leaflet_markercluster.js due to changes in core regarding multiple settings being passed.
Tested with Leaflet 0.6.4 and the matching markercluster library version.