BUG: Displacements field interpolator is broken

[oesteban]

This code:

nitransforms/nitransforms/nonlinear.py

Lines 195 to 207 in 5647caf

	new_map = np.vstack(
	tuple(
	map_coordinates(
	self._field[..., i],
	ijk,
	order=3,
	mode="constant",
	cval=np.nan,
	prefilter=True,
	)
	for i in range(self.reference.ndim)
	)
	).T

seems to be returning very wrong values.

Instead of stacking three interpolations, we should run a single 4D interpolation.