Make top_radius the top for PositionCone3dModifier

[NiseVoid]

I assume the top_radius is meant to be at the higher Y coordinate, this PR makes it so that that's true.

[NiseVoid]

.init(PositionCone3dModifier {
    height: 30.,
    top_radius: 0.,
    base_radius: 5.,
    dimension: ShapeDimension::Volume,
    speed: Value::Single(0.),
})

now results in:

[NiseVoid]

As discussed in #95, a change to the fieldname top_radius is probably the better solution to the confusion around top_radius facing down.