That way, we can use 'if constexpr', and get suitable
writing methods for all kinds of TargetSpace, without
ugly preprocessor tricks.

'Only' a stupid typo: '=' instead of '+='.

As of version 2.6, dune-functions doesn't properly do interpolation
of vector-valued functions with scalar-valued bases anymore.

The code it contains is adolcnamespaceinjections.hh.

In this case, two different refinements of the grids are not
necessarily geometrically identical.  This means we cannot
use hierarchic search with the global position of a quadrature
point, and expect to find the point on the other grid as well.

Instead, we go done to the coarsest grid, and make the transfer
to the other grid there.

Capturing any polymorphic object by value is wrong,
and gcc 8 started to warn about this in the case
of exceptions.

The latter is deprecated as of dune-functions 2.6.

This is needed to build with a recent dune-common and dune-fufem

Previously, the error norms where computed in the respective global
embedding space of the TargetSpace.  For the case of Rotation<3>
this was the space of quaternions, because this is how the
Rotation class is implemented.  However, this does not lead to
correct results.  Therefore, this patch adds special handling
for Rotation<3>, and computes the errors norms in the space of
3x3 matrices.

The CI system found this for me, because it only goes wrong
when dune-foamgrid is not available.

In many cases it is not needed, but I am too lazy to write
a proper test for it right now.

gcc warns about some of those nowadays.

It used to be in dune-functions originally, but got moved to
dune-common eventually.  This patch updates the code to reflect
that move.

Otherwise some dune-common stuff will not work properly,
for example copying of FieldMatrices that use adouble
as their number type.

The method indexSet has been removed from dune-functions
a long time ago.

It has been removed from dune-functions in the last release
(at least).

I want the interpolation rule to appear in the log files.
It is to easy to use the wrong one by accident.

Otherwise certain matrix-copying methods will not work anymore.