Properly taking into account the two-sheet covering structure

[[Imported from SVN: r8370]]

[[Imported from SVN: r8368]]

Now it properly handles the situation that p and q may be on
different sheets of the double cover.

[[Imported from SVN: r8367]]

[[Imported from SVN: r8366]]

This is safer, because calling the constructor implicitly
can lead to unwanted quaternion normalization.
(happened to me...)

[[Imported from SVN: r8344]]

Even though multiplication of Rotations is the same thing
as multiplication of Quaternions, in the first case the
output is a Rotation class, and in the second on a Quaternion
class.

[[Imported from SVN: r8343]]

[[Imported from SVN: r8342]]

[[Imported from SVN: r8333]]

[[Imported from SVN: r8329]]

Quaternion class to the Rotation class.  Rotation axes and
angles are a genuine rotation concept and don't really
belong into a quaternion implementation.

[[Imported from SVN: r8207]]

It makes it difficult to be sure that an explicitly
requested type is really handed over to every place.

While we are at it: reverse the order of the template
parameters.  Now first comes the type, then the dimension.
This is more like FieldVector and friends, and hence
should be less confusing.

[[Imported from SVN: r8148]]

[[Imported from SVN: r8146]]

[[Imported from SVN: r8115]]

[[Imported from SVN: r8111]]

[[Imported from SVN: r8110]]

[[Imported from SVN: r8105]]

[[Imported from SVN: r8104]]

fix interpolation-bug when quaternions don't have the same sign. Thanks to Oliver Sander for the patch ;-)

[[Imported from SVN: r8102]]

[[Imported from SVN: r8101]]

[[Imported from SVN: r8058]]

[[Imported from SVN: r8034]]

[[Imported from SVN: r8020]]

[[Imported from SVN: r8010]]

[[Imported from SVN: r8009]]

[[Imported from SVN: r8005]]

[[Imported from SVN: r8004]]

[[Imported from SVN: r8003]]

[[Imported from SVN: r7984]]

add method to transform an embedded tangent vector into the corresponding SkewMatrix and use this method in the old tangentToSkew method

[[Imported from SVN: r7971]]

[[Imported from SVN: r7958]]

[[Imported from SVN: r7956]]

Add the cayley and inverse cayley mapping. By now these formulas use the matrix representation of a skew matrix/rotation which is not very pretty. I will replace it by a quaternion counterpart when I figured out how to do it properly 

[[Imported from SVN: r7943]]

[[Imported from SVN: r7933]]

[[Imported from SVN: r7932]]

[[Imported from SVN: r7931]]

And use it to implement the Rotation<3> class.  This makes the
Rotation implementation easier to understand.  Previously we
have used a FieldVector<3> when a skew matrix was meant.
This leads to confusion now that we also use quaternions as
tangent vectors of SO(3).  In local coordinates, these also
have 3 entries.  Adding the new skew matrix class makes it
clearer what kind of mathematical object is meant.

[[Imported from SVN: r7897]]

It seems dangerous: I tend to get confused what these arguments
are.  Plus, the method is unused.

[[Imported from SVN: r7890]]

It's result is not necessarily a Rotation, hence such a method
would better be placed in the general 'Quaternion' class.

Also, nobody in this module used it anyways.

[[Imported from SVN: r7883]]

[[Imported from SVN: r7568]]

[[Imported from SVN: r7563]]