ENH: allow "none" as time range specification (issue #1128)

- this corresponds to 'never match', which may be useful in combination
  with -constant selection.

  Eg,

      surfaceMeshTriangulate -constant -time none

  selects only the constant entry and suppresses any automatic time loop

STYLE: adjust help for the standard -times option

- indicate that times can be comma or space separated, since this is
  otherwise not apparent. Don't mention semicolon separators in the help
  since that just adds even more clutter.