In particular:
* Forward declaring ProductManifold in the `impl` namespace
  is not a good idea.
* Add missing headers

targetspacetest.cc is extended to test for the first issue.

Using FieldMatrix<,1,1> for scalars has been the standard Dune way
for a long time, but since dune 2.7 using scalars directly works
just as well.

They are getting removed now for good after the release of
dune-fufem 2.9.  The code that replaces them has been there
for quite a while, and therefore we can simply use it
without having to worry about compatibility with older
dune-fufem versions.

This fixed the following error when building with GCC 12.1.0-8 (Debian):

```
/src/dune-gfe/dune/gfe/rotation.hh:225:42: error: invalid declarator before 'axis'
  225 |     Rotation<T,3>(Dune::FieldVector<T,3> axis, T angle)
      |                                          ^~~~
/src/dune-gfe/dune/gfe/rotation.hh:225:41: error: expected ')' before 'axis'
  225 |     Rotation<T,3>(Dune::FieldVector<T,3> axis, T angle)
      |                  ~                      ^~~~~
      |                                         )
```

Only include the multiplication of a ScaledIdentityMatrix with another FieldMatrix if dune common is older or equal 2.8

This triggers bugs in the Rotation and RigidBodyMotion classes.

In particular, it triggers what Jonathan Youett fixed in the
never-merged merge request

  !2

These bugs are fixed in this commit, too:
* The RigidBodyMotion class gets a `log` method
* The Rotation<3>::log method now returns EmbeddedTangentVector
  instead of TangentVector.

It was TangentVector whereas it should have been
EmbeddedTangentVector.

Calling log(a,b) returned a-b instead of b-a.

Also, enable unit tests for the RealTuple class.  The current test
will not trigger the bug in the log method: that will come shortly.

Finally, this commit is the first that mentions its user-facing
changes in the newly introduced CHANGELOG.md file.

The compiler warns about &, and I think the author's intention
was && anyway.

This involves fixing the secondDerivativeOfDistanceSquaredWRTSecondArgument
method: Previously it returned a FieldMatrix, but the test requires
a SymmetricMatrix.

AlexanderMueller authored 3 years ago and Sander, Oliver committed 3 years ago

This commit adds the energy implementat for the nonlinear Reissner-Mindlin
shell model known as the Simo and Fox shell model.

Details may be found in

  "A consistent finite element formulation of the geometrically
   non-linear Reissner-Mindlin shell model [Müller, Bischoff]"

Move the polar decomposition to a separate file and add the algorithm by Higham and Noferini (from Robin Fraenzel)

In the test for the Higham and Noferini algorithm, we see:
Unfortunately, for matrices that are close to an orthogonal matrix, this algorithm is
about 2x slower. One can benefit from the Higham and Noferini algorithm only if the
matrix is quite far away from an orthogonal one.

Before, the calculation was terminated after exactly three iterations, now the
stopping criterion is fulfilled if the change in the norm of the polar factor is
small enough.

Correct the sign of the Neumann and volume forces to make it consistent throughout dune-gfe and dune-elasticity

In the energy file, the energies are *added*.
The Neumann and volume forces get the correct sign in the actual program file by mulitplying with (-homotopyParameter).

The contravariant base vectors were not calculated correctly.
The contravariant base vectors are the *columns* of the inverse of the covariant matrix, not the rows.
To fix this, take the rows of the transpose of inverse of the covariant matrix.