This will later simplify merging the mixed and non-mixed versions
of the assembler code.

The ProductManifold class generalizes RigidBodyMotion, and can do
everything that the RigidBodyMotion class can.  Therefore there is
no point in keeping RigidBodyMotion any longer.  Having two
implementations for the same thing will just confuse people.

This is needed to actually compare the 3D-model with the 2D-approximation

Scalar bases used to work for that, but that feature got
removed a long time ago.

This is needed to test against the 2.6 core modules.

Using LocalIndexSet objects was mandatory with dune-functions-2.5
and earlier.  However, we only support 2.6 and later anyway,
so let's get rid of the backward-compatibility code.

That's already done in adolcnamespaceinjections.hh, in dune-fufem.

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.

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

In particular, assemblers now hand LocalView objects around, rather than pairs of
elements and LocalFiniteElements.

[[Imported from SVN: r9997]]

[[Imported from SVN: r9913]]

[[Imported from SVN: r9912]]

[[Imported from SVN: r9877]]

That should be no problem now that we use a 50-decimal digits type
for the FD approximation.

[[Imported from SVN: r9876]]

[[Imported from SVN: r9875]]

It offers a 'double' interface, but we don't really need that.

[[Imported from SVN: r9874]]

[[Imported from SVN: r9873]]

[[Imported from SVN: r9872]]

[[Imported from SVN: r9870]]

This code compares scalar and vector AD, and a finite difference
approximation.

[[Imported from SVN: r9869]]

[[Imported from SVN: r9681]]

[[Imported from SVN: r9680]]