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.