feature request, importers.importStep to return assembly if STEP file contains assembly

Currently i am loading up assembiles using this class method
```python
assembly=cq.Assembly().importStep('assembly_step1.step')
```
I normally load up step files using this importStep method
```python
workplane = importers.importStep('assembly_step1.step')
```

I would ideally like to have a single interface that loads the step file and returns an assembly if the STEP is an assembly but returns a workplane if not. Or in other words it would be useful if the following asserts were true.

```python
import cadquery as cq
from cadquery import importers
result = cq.Workplane().sphere(5)
assembly = cq.Assembly()
assembly.add(result)
assembly.export("assembly_step.step")
result.export("workplane_step.step")
loaded_object = importers.importStep('assembly_step1.step')
assert isinstance(loaded_object, cq.Assembly())
loaded_object = importers.importStep('workplane_step.step')
assert isinstance(loaded_object, cq.WorkPlane())
```

This is because I don't have a good efficient way to tell if the STEP file is an assembly or not