Addition of new entities / suffixes

[Lestropie]

This BEP will necessitate the addition of new entities to the BIDS specification.

Currently (assuming #46), this would be entities model, param and stat, and suffix model (not particularly happy with having the same string appear as both entity and suffix; open to alternatives).

So this requires:

1. Addition of these codes into the appendices at the appropriate locations.
2. Appendices must include some form of toggle to indicate whether entities / suffices are applicable to only raw data, only derivative data, or both.

[Lestropie]

Note: In current bep-016 code, "_desc-" entity is regularly used to reference the particular model being represented. I don't think it's sustainable to continue using such a generic entity for something so core to the BEP. Hence I think a dedicated entity is required to nominate the model being represented.

[Lestropie]

Following #92, there will be no new suffices (due to adoption of "_dwimap") following bids-standard/bids-specification#1602).

For entities, currently "_model-" and "_param-" are used. These will need to be added to the schema as required (I've never gone through that process so will have to learn as I go).