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Resolve "Automatically format generated C++ code using clang-format"

Closes #177

See merge request !430

Merge branch '177-automatically-format-generated-c-code-using-clang-format' of ssh://gitlab.dune-project.org:22022/extensions/dune-codegen into 177-automatically-format-generated-c-code-using-clang-format

The generated code depends on a certain include order that can be messed
up by re-ordering the includes.

* Accumulate code into single string.
* Format string with clang-format and write its output into the file.
* Use '.clang-format' style specification in top-level project source
  dir and use 'Mozilla' style as fallback.

Use driverblock option for choosing between matrix-free/matrix-based solver

See merge request !428

Driver generation for examples with matrix-free preconditioners

See merge request !427

Feature/bsg update preconditioner

See merge request !422

Feature/various fixes v2

See merge request !429