Issue 1453: MDS: fall back to SVD when the full similarity matrix is known

The SVD method can now be used to calculate the metric MDS. In order to do this a new argument ``method`` which defaults to ``smacof`` and can be set to ``svd`` was introduced in the MDS estimator.
This was done in order to fix issue #1453.
BTW: I don't understand why the method is called SVD if we don't internally use an SVD solver but instead use an eigen decomposition solver. Do you have a reference that motivates this way of implementing the "svd method for MDS"?