Data model

The purpose of this task is to define the basic data model we will use in GrimoireLab. This means, to determine what entities can be extracted from the different data sources we analyze. Plus, how these entities are related.

The model should be composed by two levels of data (maybe more) and will establish how entities are linked or related to each other. In a graph, entities are the nodes and relationships are the edges between these nodes. We can define three different types of entities:

* Entities without context: directly extracted from raw items (i.e commit, commit actions, issue, issue comments)
* Entities generated with context: merged entities or extracted with complex logic (i.e demography, onion model)
* Relationships: links between entities (ie. which commits fix which issues, author of an issue)

We well start defining the model of those entities without context, progressing to more complex relationships.

**Related tasks**

- [ ] Git data model (#164)