MRG: add resampling and smoothing functions

This pull request adds some simple processing functions when scipy
is available. The functions are:

* resample_from_to - resample first image into space of second
* resample_to_output - resample image into output (world) space
* smooth_image - smooth image over voxel axes

I also put in the utility functions converting between FWHM and sigma.
Yes, true in general I agree, but here I'm just using it for a simple 4x4 matrix inversion, and it would be a bit inconvenient to add the conditional stuff about scipy being present to avoid import errors when it isn't installed.