DOC: extend / format comments; rearrange code
DOC: extend / format comments; rearrange code

For fiac_example

Should be no functional changes.

alexis-roche/register,nipy/nipy-labs,nipy/nireg,nipy/nipy-labs,arokem/nipy,nipy/nireg,alexis-roche/nipy,alexis-roche/niseg,alexis-roche/nipy,arokem/nipy,bthirion/nipy,bthirion/nipy,alexis-roche/nireg,bthirion/nipy,alexis-roche/nipy,bthirion/nipy,alexis-roche/nireg,alexis-roche/niseg,alexis-roche/nipy,arokem/nipy,arokem/nipy,alexis-roche/register,alexis-roche/register