Add testing datasets with more complex relationships

[ManuelAlvarezC]

The current dataset we are using for unittesting is quite simple, and I'm afraid some issues may arise when working with datasets with more complex relations.

My proposal is to add datasets with:

• A single parent and multiple childs
• A child with multiple parents
• Multiple multi-level relations (a child whose parents are also parents of some of the parents of the child,...)

This would help us to catch edge cases we may have not considered.