Add first round of debug logging to UDPOR

Add first round of debug logging to UDPOR

Several XBT_DEBUG() statements were added throughout
the functions in UdporChecker to better understand
the evolution of UDPOR in the context of SimGrid.
Several `to_string()` statements were added where
appropriate to aid in the visualization of the
unfolding structure. Until we potentially add
a more complicated tree-drawing algorithm to SimGrid
that can render the unfolding structure systematically,
this is the best we have