Better handling of iterators by izip

[ramhiser]

When izip is applied to objects of class iterator, the results are a bit confusing and are quite different from Python's itertools.

R version:

> it <- izip(islice('ABCDEFG', 2, 4), islice('ABCDEFG', 1, 3))
> nextElem(it)
 Error: StopIteration

Compare with the Python version:

>>> from itertools import izip, islice
>>> it = izip(islice('ABCDEFG', 2, 4), islice('ABCDEFG', 0, 2))
>>> it.next()
('C', 'A')
>>> it.next()
('D', 'B')
>>> it.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

[ramhiser]

Similar to #30, the above R example is actually working as expected because character vectors in R are not iterables.

Consider the following R example.

 > it <- izip(islice(1:10, 2, 4), islice(1:10, 1, 3))
 > nextElem(it)
 [[1]]
 [1] 2

 [[2]]
 [1] 1

 > nextElem(it)
 [[1]]
 [1] 3

 [[2]]
 [1] 2

 > nextElem(it)
 [[1]]
 [1] 4

 [[2]]
 [1] 3

!> nextElem(it)
 Error: StopIteration