reduction_op with axis + keepdim

[mtar]

Description
A clear and concise description of the bug and the associated functionality.

reduce_op does not work when the axis argument given to a reduce_op function is smaller than the DNDarrays split axis by one.

To Reproduce
Steps to reproduce the behavior:

1. Which module/class/function is affected?
   _operations.reduce_op and functions calling it (sum, max, min ...) respectively.
2. What are the circumstances under which the bug appears?
   axis == split-1 + keepdim == True
3. What is the exact error message / erroneous behavior?
   Two processes:

[1,0]<stderr>:  File "mpi4py/MPI/Comm.pyx", line 652, in mpi4py.MPI.Comm.Allgatherv
[1,0]<stderr>:mpi4py.MPI.Exception: MPI_ERR_TRUNCATE: message truncated

Hangs on more processes.

Expected behavior
A clear and concise description of what you expected to happen.

Return DNDarray

Illustrative
If applicable, add screenshots or minimal examples to help explain your problem.

ht.sum(ht.ones((3,3), split=1), axis=0, keepdim=True)
ht.max(ht.ones((3,3,3), split=1), axis=0, keepdim=True)
ht.sum(ht.ones((3,3,3), split=2), axis=1, keepdim=True)

Version Info
Which version are you using?
master

Additional comments
Any other comments here.