[API]: changed body mapping functions: removed most of them, added mapping-decider function, accomodated changes to test cases, added new mapping test case

CMAKE_FIND_ROOT to be able to find external libraries like openmesh
CMAKE_INSTALL_PREFIX for a possible later "make install"

Updated conda packages to cond build > 3.1.4 using host section
See https://conda.io/docs/user-guide/tasks/build-packages/define-metadata.html#host

Resolve "Add builds for GCC 7"

Closes #24

See merge request walberla/walberla!37

- fixed problem with new conda by using new compiler mechanism
- added test file that is checked before upload
- open mesh recipe fixed
- force downgrade for conda-build, new tested version(3.2.1) of conda-build leads to an empty
  package and then failing test.. perhaps it is fixed with newer conda-build versions?

Enable dynamic refinement for coupled LBM-PE simulations

See merge request !73

Added discrete particle method coupling

See merge request !72

Added a showcases folder

See merge request !71

Add mesh module to waLberla

See merge request !70

Pe coupling refinement fixes

See merge request !67

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