Add optional filter method for which entities to include/exclude in a cluster

I have recently started using the clustering feature, which was easy to get working and works great. However, one use case that would be useful would be to be able to optionally say which entities should or should not be clustered (e.g., it would be nice to remove the selected entity from a cluster).

Start by locating Cesium's clustering feature and the code that selects entities for a cluster. Define an optional inclusion/exclusion filter, verify that the selected entity can be omitted without changing existing behavior when no filter is supplied, and check the clustering API documentation for the appropriate integration point.

Written by the indexing model from the issue text.