Adding +/- operators for c++ vector iteraors

Adding +/- operators for vector iterator allows the use of stl algorithm like "partial_sort":

```cython
from libcpp.vector import vector

ctypedef vector[float] farray
ctypedef vector[float].iterator fiter

cdef extern from "<algorithm>" namespace "std":
    void partial_sort(fiter, fiter, fiter)    

def my_partial_sort(np.ndarray[float, ndim=1] x, int k):
   cdef farray arr = x
   partial_sort(arr.begin(), arr.begin()+k, arr.end())
   x[:] = arr
```
The += operator works properly but it leads to some problems when using it in a prange loop since the variable becomes a reduce variable.