This merge requests adds test for the matrix-free preconditioners. The test solves the Poisson problem and covers regular and fast DG grid operator. The problem is solved in a matrix-free and a matrix-based version and the number of iterations are compared.
Note: For this test it made sense to move the tests from dune-codegen/test/sumfact/preconditioner to dune-codegen/test/matrix-free-preconditioner. By moving this file I could avoid duplicating the test driver file and have all tests for the preconditioners in one place.
