Because this object doesn't exist in dune-functions bases anymore.

[cleanup] Move the FD approximation of the GFE function gradient from the GFE function into the test

It is a part of the test harness, and should not clutter the implementation of the
GFE functions themselves.

Use 1e-8: The square root of the machine precision is said to be the best choice.

Needed for projection-based finite elements.

This patch also adds a unit test.  So far this test passes.

PQKNodalBasis has been renamed to PQkNodalBasis

The program is not really a test, but it doesn't really belong into 'src', either.

Unfortunately, this bases needs to be switched on and off using preprocessor
commands.  Hopefully I'll find a way to make that prettier in the future.

It was only in the dune-gfe module for historic reasons.

The inline expression in the parameter file that we used previously is
too short and limited.  In particular, we want to be able to use the
same expression for error measurements.  However, for that we need
a function with a derivative.