Fixing `DMCSimulationResults.flat_result_iter`

Fixing bug to method introduced in commit
9b0dc7aa254d52f2ff82ecd41ce246ad77b9b853. The assignment of new
key/value pairs to the flat dictionary of results when
`include_tau_exclusion_info` was `True` was broken; it was attempting to
append the values to a list referenced by the key, which did not exist.