Keep track of past participations in Clusters

[rgaudin]

Because mapping applies to all times, GeoJSON should return a list of all the features that has ever been used within a Cluster.
Current behavior is to return the members of the Cluster at that time.
We don't have missing features just now because nobody has ever left the Cluster but it might occur in the future.
We need to store a unified collection of:

• Entities ever involved (had a participation or ExpectedReporting?)
• Are in the Cluster without ExpectedReporting (NOT_EXPECTED)