Added Barnes-Hut tree N-body code using a phtread parallelisation

See merge request !96

 * fixed clang warnings

  * fixed warnings

 * fixed clang warnings

  * fixed clang warning

 * optimized Node::sufficientForBody

  * delete dead code
  * fix further cppcheck hints

  * find useful names

  * (added boost dependency)

  * increase number of iterations to get reasonable long runtime

  * use std::all_of Tree::isCorrect()
  * refactoring