quickstart tutorial has traceback

[Greg Couch]

Doing the volume example, after executing:

   fit #2 in #1
   volume #1 transparency 0.5

I get:

Traceback (most recent call last):
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/ui/gui.py", line 462, in customEvent
    func(*args, **kw)
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/ui/widgets/htmlview.py", line 247, in defer
    cxcmd(session, topic)
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/ui/widgets/htmlview.py", line 344, in cxcmd
    ti.execute()
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/cmd_line/tool.py", line 205, in execute
    cmd.run(cmd_text)
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/core/commands/cli.py", line 2634, in run
    result = ci.function(session, **kw_args)
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/core/map/volumecommand.py", line 327, in volume
    apply_volume_options(v, dsettings, rsettings, session)
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/core/map/volumecommand.py", line 352, in apply_volume_options
    kw = level_and_color_settings(v, doptions)
  File "/home/gregc/src/chimerax/ChimeraX.app/lib/python3.6/site-packages/chimerax/core/map/volumecommand.py", line 495, in level_and_color_settings
    clist = [colors[0].rgba]*len(getattr(v, style + '_levels'))
AttributeError: 'Volume' object has no attribute 'surface_levels'

[Tom Goddard]

Fixed.

Removed volume surface_levels attribute yesterday but missed this tricky getattr() use of it.