Merge pull request 'feature/update_readme' (#1) from peters/VirtualFluids_dev:feature/update_readme into open_source

Adds authors and contributing file. Clean up the readme file.
Set the minimum cmake version to 3.13 according to the readme entry.
Set the options BUILD_VF_CPU, BUILD_VF_GPU and BUILD_VF_UNIT_TEST to OFF by default.

* feature/updating_googletest:
  Set CMake Output directories for runtime, library, archive and pdb files for all libraries and executuables.
  Supress gtest rpath warning and remove install option.
  Added googletest untracked files. Adjust 3rd/gmock.cmake file.
  Update Googletest to v1.10.0. Put 3rdParty targets in msvc subfolder "3rd".

Set CMake Output directories for runtime, library, archive and pdb files for all libraries and executuables.

Warning: [-Wbackslash-newline-escape]
https://stackoverflow.com/questions/31673945/clang-backslash-and-newline-separated-by-space-in-single-line-comment

* feature/adaptToOpenSource: (1091 commits)
  Add GPU Open Souce Solution to the new Cmake Layout.
  Remove non open source app.
  Add CPU Open Source App.
  Moved VirtualFludis Open Source to the new cmake structure.
  Removed unused basic source code. Moved numers/geomtry3d to /geometry3d.
  Removed old export include header. Little clean up in cmake.
  - add metis path to config file
  - Refactoring in vf_add_library and more...
  Small clean ups for the test executable.
  Small clean up.
  Added a little bit documentation and formatting.
  Clean up the starting cmake file
  Adapt export statements in the gridgenerator.
  Removed complete boost dependency in basics.
  Added ToIntVector Test and removed the boost foreach.

  Added unit test for endsWith() and removed the boost dependency.
  Added unittest functionality.
  Add tesla01 config file
  Changed VF_PUBLIC define to VIRTUALFLUIDS_GPU_EXPORT according to new export library naming scheme.
  see last commit
  ...

Moved numers/geomtry3d to /geometry3d.