Add "part" entity for magnitude and phase data

[tsalo]

I would like to add support for magnitude and phase data across modalities (except func, as a new suffix was added there in #128). This new "part" entity is part of BEP-001, but it is applicable to more than the modalities covered in that extension, including SWI (see the "part" entity proposed in BEP-004).

I think it would be a good idea to add the entity to the main specification outside of any extension proposal, which will also make BEP-001 easier to review when the time comes.