Suggestion to add 'asp' argument to scatterplot3d()

I need to produce 3-d scatter diagrams with equal aspect ratio for
all axes.  However, scatterplot3d() currently does not allow setting
aspect ratio. I try to circumvent this by setting equal x|y|zlim,
but this is only approximate and depends on the shape of plotting
device. This patch suggests a minimal change, and default behaviour
of scatterplot3d() is unchanged.
set 'asp' also when evaluating string widths
adapt axis annotations to 'asp'
axis annotation adjusted correctly also for asp <> 1
add closing }
Merge branch 'set-asp-ratio'