issue_comments: 287673906

I would also like to see an integrate function. I have had one monkey patched in my own xarray routines for a while now. Also wanted: cumtrapz and friends. Maybe this could be implemented by adding an optional cumulative flag. This shouldn't be too hard to do. For example, in the following cumtrapz implementation all that would need to be changed is the final cumsum call.

```python def cumtrapz(A, dim): """Cumulative Simpson's rule (aka Tai's method)

Notes
-----
Simpson rule is given by
    int f (x) = sum (f_i+f_i+1) dx / 2
"""
x = A[dim]
dx = x - x.shift(**{dim:1})
dx = dx.fillna(0.0)
return ((A.shift(**{dim:1}) + A)*dx/2.0)\
      .fillna(0.0)\
      .cumsum(dim)

```