Improving the import & export format of unary relations

[Laschoking]

The format attribute requires a tuple. As a special case, if only one type is given the pending , could be left out for simplicity.
@import Test :- tsv{resource="Test.tsv",format=(string,)
proposed:
@import Test :- tsv{resource="Test.tsv",format=(string)

[matzemathics]

As far as I can see, there are two options:

• accept (expr) as syntax for one-tuples -- this will require further consideration, probably we want something along the lines of "only interpret (expr) it as a tuple, if it is not sub-expression of another expression"
• accept non-tuples in the format argument

The first one seems harder to implement, but also cleaner conceptually. I might give it a shot.

[matzemathics]

After examining the code and thinking about it some more, it seems hard to find a consistent rule, how to handle unary tuples. Maybe the second option is simply more consistent.