MISC: index with float is deprecated in numpy
DOC: misc naming change

This is more of a tutorial than a user guide
DOC: alphabetic ordering of reference
DOC: Fix see also
DOC: numpy doc standards

The return was not displayed in the doc, as the required section title is
'Returns'
DOC: fix rst in docstring
DOC: add nisl.image to reference doc
API: smooth: param renamed to fwhm