Fix two minor bugs in the ODPOR implementation
This commit fixes the following two bugs:
1. When seeding a wakeup tree for the first
time, we forgot to add a `break` statement
to ensure that only one such enabled thread
were placed into the wakeup tree. Before
all such enabled threads were inserted which
amounted to what would have been a brute-force
search :(.
2. When determining whether a sequence is a
candidate for insertion into a wakeup tree,
we may need to check, for all enabled actors
in a given state, whether those actors'
next steps are independent with the sequence
theretofore constructed. This additional
step peforms the work needed to check whether
those actors are contained in the set of
*weak* initials. The method corresponds
to line 6 of the pseudocode of the ODPOR algorithm
