Add logic for subtree extraction from wakeup trees
Extracting a subtree from a wakeup tree is required
for ODPOR to continue its search and to pass information
down to the other paths that ODPOR decides to visit.
Extracting a copy of a subtree is difficult as it
requires a deep copy of the tree, making sure
that references to any nodes in the tree that is copied
are copied appropriately. One additional subtlety is that
the subtree can only be taken with respect to single-process
branches and, further, that that process is removed from
each descendant of the node that contains the single process.
