Added a showcases folder

See merge request walberla/walberla!71

Add mesh module to waLberla

See merge request walberla/walberla!70

Pe coupling refinement fixes

See merge request walberla/walberla!67

[API]: changed pe coupling mapping functions interfaces, added new mapping functions, adapted test cases and benchmarks

Streamed GPUPackInfo

Closes #27

See merge request walberla/walberla!68

- one python file can now generate multiple source files

 lbmpy boundary handling works on pure flag fields, geometry setup
 classes could only set up boundary handling, not pure flag fields.

Changes introduced in this commit are the following:

- CUDA streams: Add support for asynchronous (un)packing operations using CUDA
  streams in cuda::communication::GPUPackInfo. Through asynchronous operations
  it is possible to overlap GPU computation and MPI communication in simulations
  (e.g. LBM simulations). Asynchronous copies in CUDA require pinned memory on
  the host, and for that purpose a staging buffer is introduced (i.e.
  cuda::communication::PinnedMemoryBuffer) in the cuda module, which is used to
  stage data between the GPU and the MPI buffers.

- zyxf layout: Add zyxf field layout support in GPUPackInfo through extensions
  of the functions in cuda::GPUCopy.

- Extended GPUPackInfo test: Add stream and zyxf layout tests to the
  GPUPackInfoTest to test the proposed implementation.

- Extended Kernel: add CUDA stream and shared memory configuration support in
  cuda::Kernel class.

Signed-off-by: João Victor Tozatti Risso <joaovictortr@protonmail.com>