NF: error if time diff slice axis is time axis

Running time_slice_diff on an array allowed the slice and the time axes
to be the same.  Test that they aren't.
NF: function giving time difference for images

New function that accepts an image as input and returns time series
difference analysis on the image.
WIP: refactor tsdiffana script to select slice axis
WIP: refactoring diagnostics commands into modules
RF: refuse to load / convert MINC for now

Nibabel can load MINC, but now we need an API to get the axis names for
MINC, otherwise, the axes can be anywhere, but are most likely to be in
the opposite order to the ones the NIfTI needs.  Raise an error until we
have worked out how to do this right.
TST: skip test for non-analyze types for now

In fact it turns out we can't sensibly load the non-analyze types, so
skip this test.
TST: develop tests for tsdiffana command function

Use command function to test the tsdiffana interface.
TST: extend tests for nipy_tsdiffana script

Smoke test for time and slice axis command line options.
NF: tsdiffana option to save results volumes

Add option for tsdiffana to be able to save the results volumes created
during the analysis.
BF+TST: test+fix bugs for PCA on reordered images

The `screen` and `write_screen_res` functions were assuming time-last
ordering in a couple of places.  Test and fix.
RF+TST: refactor nipy_diagnose into module, test

Put logic for nipy_diagnose into commands module.

Use time and slice axis testing function developed for tsdiffana.

Extend tests of diagnose outputs.
RF: deprecate guessed slice axis for screens func

The command line utilities nipy_tsdiffana and nipy_diagnose guess the
slice axis for Analyze-like files (like Nifti) - so the `screens`
function doesn't need to do that, and it doesn't match the API for --
say -- `pca_image`.  So deprecate the slice axis guess in `screens` and
test.
Merge pull request #308 from matthew-brett/refactor-image-time-diff

MRG: refactor of time slice routines

I hadn't really thought through selecting the slice axis for time-slice
routine utilties or for the diagnostic screening utilities.

Refactor the nipy_tsdiffana and nipy_diagnose command line utilties to
use a common interface for selecting slice and time axes. Put command
logic in a commands module and test more. Generalize diagnostic
screening utilities to any time axis rather than assuming time is the
last axis (the pca module explicitly allows this). Deprecate guess at
slice axis for screening function (but continue to guess for Nifti-like
images from the command line).