Avoid otherwise integrals

See merge request !436

(cherry picked from commit 29ae0358)

e7a0b05e Import from collections.abc
758b5d12 Avoid generation of redundant 'otherwise' integrals

Co-authored-by: Dominic Kempf <dominic.kempf@iwr.uni-heidelberg.de>

Add new vectorization strategy visualization

See merge request !432

Additional coefficients with fast-DG

See merge request !431

Correctly handle additional coefficients in the fast-DG case. The
previous code was also working but it copied the degrees of freedom of
the additional coefficient vector into a local vector instead of
directly using the global data structure.

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