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.