B-Spline mapping of individual points is broken

[oesteban]

My plan is to merge #138 knowingly that the BSplineFieldTransform.map() method is broken, as reflected by the doctest (the outputs of this doctest should be very close to those of the displacements field transform).

Given the complexity of the problem, I believe it is worth integrating the base infrastructure for B-Splines and then think about this issue.