Include common configurations within the package

Having to copy the contents, or download a link like `https://raw.githubusercontent.com/BlueBrain/nexus-forge/master/examples/notebooks/use-cases/prod-forge-nexus.yml` is cumbersome to make basic queries. Adding the file to the package when being deployed, and having a method to get them would make things much simpler.

Perhaps something like:

```
from kgforge import configurations
from kgforge.core import KnowledgeGraphForge

forge = KnowledgeGraphForge(configurations.get("BBPPublic"))
```

would make it easier to connect to instances.