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

Including updated tests.
The idea was to have a way to return "diagnostic" parameters from the B-spline implementation without specifying in advance what those are - because that will be implementation-dependent.
However: a) the method currently isn't used anywhere, so we could just remove it; b) on second thought, not coupling the wrapper class (BSpline2d) to the implementing library (eol-bspline) doesn't really work here, because the parameters in the BSpline2d constructor are specific to the library. (Which could be avoided by using Param again.) ;-)