BF - remove doctest dependency on sympy-0.7
BF - remove doctest dependency on sympy-0.7

I copied the compatibility code from sympy-0.7.  The doctests used
sympy.Tuple, which is new in sympy-0.7, so the doctests in this module
failed with sympy-0.6.6.

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