RF - docstring + minor refactoring of generators
RF - docstring + minor refactoring of generators

No functional change.

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