staggered boundary generation: direction index should point to fluid cell when inner_or_boundary=False

In that case, the index vector contains the boundary cells, so the direction should point to the fluid cell.
When inner_or_boundary=True, the index vector contains the fluid cells and the direction points to the boundary cell.

[BUGFIX] missing executables

See merge request walberla/walberla!296

Since some benchmarks/tutorials are registered as tests they have to be
built in order to run.

Allowed different field layouts in LBM codegen

Closes #119

See merge request walberla/walberla!294

Added warning if no SIMD instruction could be determined, adapted field layouts in all tests and benchmarks

FFT: show error when using with MPI_COMM_WORLD

See merge request walberla/walberla!293

enable mesa_pd codegen for ci

See merge request walberla/walberla!292

heat conduction and thermal expansion showcase

See merge request walberla/walberla!284

moved coverage image to local registry

See merge request walberla/walberla!291

Moved from heat capacity to specific heat
to take into account individual mass of a particle.

Use serial I/O instead of MPI-IO for certain versions of OpenMPI

See merge request walberla/walberla!282

removed mrt3 from each test case

See merge request walberla/walberla!287

optimize build time

See merge request walberla/walberla!290

enable clang10

See merge request walberla/walberla!288

Renamed template parameter name.

See merge request walberla/walberla!286

See merge request walberla/walberla!285

The old name (REAL) caused problems when coupling the Trilinos library
since some component has a global define with the same name.

Codegen and cuda

Closes #116

See merge request walberla/walberla!277

use cuda as a first level language in CMake with `enable_language(CUDA)`
enable tests where cuda and codegen are both activated
various minor fixes