Allowing users to specify objects . object attributes not direclty in the source

When specifying the object to ```:select:``` there is a check to see if this object is the ```source.keys()```

If the object is not in the source keys then an error is raised.
https://github.com/Peque/sphinxcadquery/blob/0b7050c0204b8b2c6f7e6912eaee97ca02b9e776/sphinxcadquery/sphinxcadquery.py#L69

In my case I have wrapped the CadQuery object with another object.

My cadquery object is not directly in the source so it is not found

For my use case it would be better if that ```RaiseError``` was swapped to a ```Warning``` as and I would be able to enter ```:select: my_reactor.solid```

For example here I have to do ```cadquery_object = my_reactor.solid``` and then set ```:select: cadquery_object```
https://paramak--836.org.readthedocs.build/en/836/example_neutronics_simulations.html#ball-reactor-ipynb

Ideally I would not include the ```cadquery_object = my_reactor.solid``` line in the example.

I admit this is quite a niche use case but something for consideration