Add first "working" version of ODPOR
This commit introduces the first version of
ODPOR which detects a bug using the example
programs found in SimGrid's test suite! There
is still work to be done (e.g. dealing with
transitions that have multiple possible
paths of execution such as MCRandom() will
be non-trivial to resolve).
The key adjustments in this commit are
to the WakeupTree structure itself. Gone
are the days when each node in the tree
maintained a sequence of transitions to
execute; instead, each node contains only
a single transition, and its the paths between
a node and the root that describe the partial
executions that ODPOR needs to explore.
This resulted from a misconception I had about
the structure of wakeup trees. When an insertion
is made, potentially a *chain* of leaf nodes
are added into the tree. This ensures we always
have a single-process branch to pick (which
is what we expected) and greatly improves the
readability of some of the methods found in
State.cpp related to ODPOR
