Remove unnecessary numpy usage.

These simple loops can be achieved in pure Python and numpy doesn't
really create any benefit here.
Remove unnecessary calculations.

x_slice and y_slice are only used to find the one process's cpux and
cpuy. That can be done with a simple calculation just like in the
non-MPI case.
Optimize numpy usage a bit.

* Don't use zip() because that converts from a numpy array to a regular
  Python list. And then we convert back to a numpy array anyway.
* Don't use numpy.append(). It's better to pre-allocate the entire array
  to its final size first.
Merge pull request #4 from QuLogic/numpy

NumPy cleanup