I've added the cluster system building and solving.
I've also added the computation of the relative aberration of a cluster.
Deleted the PvmClusterDataCoreSplitter, as its functionality would be better
integrated directly into the PvmClusterDataCore class.
I've also finished writing the function for splitting a cluster.
Finished the whole cluster feature, but did not get a chance to test it properly.
Integration tests should be written to test that the result complies with the intended
feature characteristics.