provide better default values in MeshIO.read_times()
update file sources for providing both steps and times

- update FileSource.reset(), .get_step_range()
- remove FileSource.set_step(), VTKFileSource.get_step_range(),
  VTKSequenceFileSource.get_step_range(), GenericFileSource.get_step_range()
  GenericSequenceFileSource.get_step_range()
- update GenericFileSource.__init__(), .read_common(), .file_changed()
- new FileSource.get_step_time(), .get_ts_info()
- new VTKSequenceFileSource.__init__()
- new GenericSequenceFileSource.read_common()
update postproc.py, Viewer for updated file sources, add slider for times

- update Viewer.call_mlab()
- update SetStep:
  - new ._time_editor, .time, .time_low, .time_high, .is_adjust
    ._time_changed()
  - update .__source_changed(), ._step_changed(), ._file_changed_changed()