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.

Make them a parameter instead.  This allows to switch more easily between
geodesic finite elements and projected finite elements.

Otherwise, the ParaView 'glyph' filter won't recognize the direction fields.

Such measurements are now centralized in the program compute-disc-error.cc.

It is still no function in the dune-functions sense, but at least the
basis parameter needs to be a dune-functions basis now, rather than
a deprecated dune-fufem basis.