Package import fails if kgv3 client is not installed

[dickscheid]

When importing fairgraph withough having the KG v3 python client installed, an exception is raised because the "Stage" module will be undefined in the following line:

https://github.com/HumanBrainProject/fairgraph/blob/d5fa8ac4b6d812477a6e1ba2eec0b72f3000f942/fairgraph/client_v3.py#L44