Creating and exporting multiple objects in one 3MF file

Hello together,

I have a question about exporting multiple objects in a stl file. What i want is, when I import the exported stl-file into the Cura software, the objects should be listed individually, and when I slice it, each object should be finished one at a time, instead of adding a layer to all objects in the same iteration in the print.

my current snippet for creating simple cylinder objects

```python

workplanes = []

for point in self.__create_array_points(cylinder_radius * 2, distance, quantity):
workplane = Workplane()
workplane.pushPoints([point]).cylinder(cylinder_height, cylinder_radius)
workplanes.append(workplane)

# what i want
exporters.export(workplanes, "test.stl")
```

The following issue may address the same problem https://github.com/CadQuery/cadquery/issues/212, but I was not able to achieve the result I wanted due to my lack of knowledge.