MRG: helper routines for reslicing to world space

These are some routines that don't look much, but are the basis for a very
useful feature, which is to be able to reslice an image into a voxel block
aligned to the output (world) axes.  The resliced image is one way of showing
the image in world (output) space, so it is very useful for visualization and
checking affines.
may be the header for consistency?
```
# emacs: -*- mode: python-mode; py-indent-offset: 4; indent-tabs-mode: nil -*-
# vi: set ft=python sts=4 ts=4 sw=4 et:
### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ##
#
#   See COPYING file distributed along with the NiBabel package for the
#   copyright and license terms.
#
### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ### ##
```