Class Arrow

      object --+        
               |        
common.Primitive --+    
                   |    
           ArrowCore --+
                       |
                      Arrow

Class for drawing a 3D arrow in OpenGL.  Will be drawn
in the current OpenGL drawable.  This includes the ability
to draw into Maestro's Workspace.  It just needs to be
the current drawable.

See doc string for __init__ for details on instantiation

Arrows should be added to a Group and drawing done via the Group.
See the graphics3d.common.Group documentation.

API Example
-----------
import schrodinger.maestro.maestro as maestro
import schrodinger.graphics3d.common as common
import schrodinger.graphics3d.arrow as arrow

arrow_grp = common.Group()
st = maestro.workspace_get()
for bond in st.bond:
    ar = arrow.Arrow(
        xhead=bond.atom2.x, yhead=bond.atom2.y, zhead=bond.atom2.z,
        xtail=bond.atom1.x, ytail=bond.atom1.y, ztail=bond.atom1.z,
        color='red',
        radius=0.15,
        opacity=0.8,
        resolution=50
    )
    # Add the primative to the container.
    arrow_grp.add(ar)

# Add the draw callback.
maestro.workspace_draw_function_add(arrow_grp.draw)

# Hide the markers.
arrow_grp.hide()

# Remove the markers and the callback.
arrow_grp.clear()
maestro.workspace_draw_function_remove(arrow_grp.draw)

Constructor requires:

xhead, yhead, zhead: corrdinate of the head position
xtail, ytail, ztail: coordinate of the tail position

color:    One of:
            Color object (Color class)
            Color name (string)
            Tuple of (R, G, B) (each a float in range 0.0-1.0)

Optional arguments:

radius:   radius of the arrow in Angstroms
          Default: .15
opacity:  0.0 (invisible) through 1.0 (opaque)
               Default: 0.0
resolution:    Ranges from ?? to ??.  Default: ??