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.

Add embedded interpolation option in localprojectedfefunction.hh

See merge request !109

Remove old dune-fufem bases

See merge request !108

2.7 is simply too old now.  We could even go straight for 2.9,
but let's try how far we get with 2.8.  That one is in current
Ubuntu and Debian releases.

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.

.gitlab-ci.yml: update URL of dune-elasticity repository

See merge request !106

Adjust address to dune-elasticity in .gitlab-ci.yml

See merge request !107

convert file to UTF-8 encoding

See merge request !105

Enable the proximal newton solver in simofoxshell

See merge request !102

fix build failures with GCC 12 and without IPopt

See merge request !103

use `getCommunication()` instead of `getCollectiveCommunication()`

See merge request !104

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)
      |                  ~                      ^~~~~
      |                                         )
```

[CI] Run the 'codespell' as part of CI

See merge request !101

replace `enum` with `constexpr static int`

See merge request !100

Only include the multiplication of a ScaledIdentityMatrix with another...

See merge request !99

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

Replace traceSquared(sym(.)) by traceSquared(.) as it is the same

See merge request !98

Miscellaneous modernization

See merge request !96

Enable more tests for the various TargetSpace classes

See merge request !93

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.