Re-added B-spline transformation for non-linear RT alignment in MapAlignerIdentification

Including updated tests.
@hroest: In the previous B-spline implementation (GSL), we actually added two linear models for extrapolation (using the slopes of the spline at the endpoints). In the current implementation based on "eol-bspline", there is a parameter "boundary_condition" that controls what happens at the endpoints. I assume one of the settings ("1" or "2"?) should amount to linear extrapolation, but I haven't actually tested that.