A model for clingo's statistics

As shown [here](https://github.com/Aluriak/clyngor/issues/22#issuecomment-578968899), statistics access is different whether clyngor is using clingo module or just parsing its output.

This is due to the fact that using the clingo module, clyngor uses the API objects encapsulating the final statistics, whereas without the clingo module it parses the output, building the `model.statistics` dictionnary using clingo's stdout.

In order to clyngor to reach production-grade concerning that aspect of clingo, we need to choose a data structure to encapsulate the statistics data (either a dict, or something looking like the object of clingo's API, or something else), and convert the obtained statistics to that data structure.