Change cp.sum(*iterable, **kwargs) to cp.sum(iterable, **kwargs)

[hbierlee]

Fixes #654. It is over-explained in the linked issue, but the discrepancy is simply this:

>>> cp.sum(1,2)
3
>>> sum(1,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'int' object is not iterable

Allowing cp.sum(1,2) was I think accidentally introduced by #111. I believe we should follow Python's design for all builtins, but this is a breaking change, so perhaps needs to be moved to cpmpy 0.10 / stable.