Previously, the actual projection happened by calling the constructor
of one of the classes that implements points on a manifold.  This patch
introduces an new method projectOnto, which makes the projection more
explicit.

Makes it faster, but only by 1 or 2 percent.

The method using the SVD works in principle.  However, it does not play nicely
with automatic differentiation, because singular values are not always differentiable.

The method proposed by Higham to compute the polar factor, on the other hand,
is a Newton method at heart.  Therefore, it can be used together with AD without
problems.

[[Imported from SVN: r10076]]

[[Imported from SVN: r10070]]

[[Imported from SVN: r10069]]

As of yet completely untested

[[Imported from SVN: r9898]]

[[Imported from SVN: r9682]]