Su Viscoelasticity

See merge request !146

Fix VTK output for bodies with infinite mass

See merge request !167

Only call MPI_Finalize if we actually called MPI_Init

See merge request !166

Call target_link_libraries with PUBLIC keyword if necessary

See merge request !164

CMake: Make doc target and additional CXX warning flags switchable

See merge request !165

Remove boost::bind from tutorial

See merge request !161

remove mscv 2015 from ci and use cuda 9.0 in ubuntu 16.04

Closes #79

See merge request !162

remove clang 3 and intel 2016

Closes #79

See merge request !160

Resolve "Replace deprecated CUDA APIs"

Closes #71

See merge request !157

- information from CMake to codegen: double/float, OpenMP, ...

- More flexibility for the user to implement custom checkpointing

MPICH2 apparently expects a non-const pointer for blocklenghts whereas
other MPIs expect const pointer -> removed const to work with all

- intel 19 internal bug when using auto, and implicit capture in lambdas

- LB on GPU, Uniform Grid Benchmark app
- helper class to schedule tasks to multiple CUDA streams

- comparison to old CPU transfer communication included

- CUDA compiler has problems to compile external json library
- timing pool is included by BlockForest, and previously timing pool
  included JSON library

previously the ghost layer was aligned

Features:
   - uses generated pack infos for packing & unpacking directly on GPU
   - can directly send GPU buffers if cuda-enabled MPI is available,
     otherwise the packed buffers are transfered to CPU first
   - communication hiding with cuda streams: communication can be run
     asynchronously - especially useful when compute kernel is also
     split up into inner and outer part

- added RAII classes for CUDA streams and events
- equivalence test that checks if generated CPU and GPU (overlapped)
  versions are computing same result as normal waLBerla LBM kernel

- templated buffer classes allow the buffer systems to work with custom
  buffers, e.g. buffers of GPU memory