Include a first BIDS dataset inheriting from `BaseBIDSDataset`

[PierreGtch]

With the new class BaseBIDSDataset (see #724), it becomes easier to include BIDS datasets in moabb.
To further facilitate their integration, we should

• integrate a first dataset which inherits from BaseBIDSDataset (or modify one of the existing datasets)
• present this first BIDS dataset as a "template" for integrating BIDS datasets in moabb. Probably add a pointer to it in the docstring of BaseBIDSDataset. Maybe even add a tutorial?

The goal of this issue is to make it as straightforward as possible for new contributors to add new BIDS datasets in MOABB