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.

274e09cd introduced a skip of the generation if the target file is not
present. While this might hint a wrong target directory it also prevents the
generation of new files. Since the provided mesa_pd.py script asks to
confirm the target directory anyway, this skip is removed.

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

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

This new function does the actual generation using pystencils and creates a target library that can be used as a dependency as usual. One should not list the python script itself anymore in add_executable.
Convention: Use the AppName as a prefix for the generated lib!

Example Usage:

waLBerla_generate_target_from_python( NAME AppNameGeneratedLib FILE GenerateKernel.py OUT_FILES Kernel.h Kernel.cpp)

waLBerla_add_executable ( NAME AppNameGeneratedLibTest DEPENDS GeneratedLib)

since std::vector<bool> behaves differently to std::vector using bool
can cause troubles

Previously, anything that wasn’t velocity, src, src_tmp, pdfs, pdfs_tmp resulted in invalid code

This was a regression introduced in a1614152

To remove code duplication the distinction is now moved
into the generator. Default kernels are "with shape".

Added hydrodynamic force notifications, introduced them to tests and benchmarks, minor documentation and include fixes

ForceTorqueNotification
HeatFluxNotification

- added ability to create custom modules