ENH: improve findInstance handling of negative times (#1112)

- normally findInstance will 'bottom out' with the constant directory
  while doing its reverse time search. This mechanism however fails
  when searching for negative start values (if there are none in the
  list). Add additional logic for this so that constant will also be
  used in these situations.

Note: to have decomposePar work for all times, may need the -constant option
to trigger the proper time list.