Up until dune-common 2.7, the Dune namespace contained shared_ptr:
First as an actual implementation, later simply as a namespace
import.  Dune-common post-2.7 has removed that import,
and users of shared_ptr really have to explicitly specify
the std:: namespace prefix.

Because it really is only an energy.

Because it is not a stiffness, it is really just an energy.

Previously, they all inherited from LocalGeodesicFEStiffness,
which was weird, because none of them implemented first or
second derivatives as you would expect from something called
'stiffness'.

Previously, the layout of my computer's file system was
hard-coded here.

In this file they are not needed at all anymore.

It does not look like that code is used anytime soon, so let's get rid
of it for the sake of tidyness.

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

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.

The latter is deprecated as of dune-functions 2.6.

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

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

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.

Makes the code a bit shorter and easier to read.