Fixed implementation of marshalling union, added test.

See merge request walberla/walberla!145

- 20 template parameter ought to be enough for everybody :P

add pretest stage to reduce jobs in pipeline

See merge request walberla/walberla!144

Resolve "Bug in getFriction function"

Closes #72

See merge request walberla/walberla!143

CI: GUI and CUDA

See merge request walberla/walberla!141

-> intel compiler went out of memory

- recent GCC version produce error when "-isystem /usr/include" is added
  to command line. Error is: '#include_next <math.h>' not found
- the removed "hack" added the cuda include dir with "-isystem" to
  the CUDA compiler arguments to prevent warnings from boost header
  -> currently there seem to be no warnings generated from boost headers
     so "hack" is removed

Add Intel 2019 compiler to CI

See merge request walberla/walberla!138

Add Intel Compiler 2018 to CI

See merge request walberla/walberla!133

Resolve "DEM benchmarking tool"

Closes #51

See merge request walberla/walberla!84

Clang tidy performance checks

See merge request walberla/walberla!137