Unable to export 3MF files without corruption

I've tried a couple of approaches, neither of which worked. I started off trying to export multi-object files. That didn't work and so too with single-object files. I'm running CadQuery 2.2.0.

# `Compound`
I've tried the `makeCoumpound` solution discussed in #1165. This produces a corrupt 3mf file that can't be opened in other programs:
* `Couldn't load that. Error code: 0x80004005` in 3D Builder
* `Cannot insert mesh body. The source file may be damaged.` in Fusion360

Here's the minimal example:

```python
import cadquery as cq

cq.exporters.export(
cq.Compound.makeCompound([
cq.Workplane("front").box(2.0, 2.0, 0.5,centered=[False,False,False]).val(),
cq.Workplane("back").box(2.0, 2.0, 0.5,centered=[False,False,False]).val()
]),
"3mf_from_compound.3mf"
)
```

I can provide the produced `3mf` file if that's helpful.

# Single-object export

Trying an even simpler example with a single object produces the same errors as above:

```python
cq.exporters.export(
cq.Workplane("front").box(2.0, 2.0, 0.5,centered=[False,False,False])
,
"3mf_from_assembly.3mf"
)
```

# `Assembly`
I've also tried to use an assembly and that produces the error `AttributeError: 'Assembly' object has no attribute 'tessellate'`. TBF, I didn't see anything that says that this is supported.

```python
import cadquery as cq

cq.exporters.export(
cq.Assembly()
.add(cq.Workplane("front").box(2.0, 2.0, 0.5,centered=[False,False,False]))
.add(cq.Workplane("back").box(2.0, 2.0, 0.5,centered=[False,False,False]))
,
"3mf_from_assembly.3mf"
)
```