DOC: update forest module to docstring standard
DOC: update forest module to docstring standard

Also make a relative import explicit.

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