Removes leading and trailing "tail" portions of each .putative_cluster based on self-crossings
of the within-cluster track. The intent is to keep only the locally revisited core of a cluster
and drop commuting legs at the both beginning and end.
Details
If the track has any self-crossing steps, the kept "core" is defined as the contiguous block of
steps between the first and last crossing; all points outside this block are set to NA in
.putative_cluster. If a cluster has no self-crossings at all, the entire cluster is dropped
(all its points are set to NA).