Relations whose primary keys have zero attributes

[austin-hilberg]

Currently datajoint prohibits creating tables with no attributes in the primary key. We can relax this prohibition.

If the primary key has no attributes, it can only have one value. Therefore, the relation can only contain one tuple. So it's good for relations that contain a single fact about the universe rather than a collection of similar facts. The relational algebra using such relations remains identical to what it is now. In particular, joining two relations with no primary key produces another relation with no primary key and one tuple.

Corresponds to datajoint-python issue #113