Add concepts for const and mutable matrix backends and create them automatically

This adds two new concepts ConstMatrixBackend and MatrixBackend which allows to follow the pattern used in dune-functions:

• Add utility functions to wrap a matrix into an ISTLMatrixBackend that does nothing if the concept is already satisfied.
• Test the new utilities.
• Use these utilities in algorithms (here: global operator assembler) to add support for passing a raw matrix or a manually created backend.
• Stop creating matrix backends manually in tests and examples using the global assembler.