* use iterators more extensively

  * do squared comparison instead of sqrt
  * remove unused Node::parent
  * forwarding reference optimization for MPI send

  * removed unnecessary this pointer qualifier when accessing members

  * namespace ending comments
  * fix compiler warnings
  * fix more clang-tidy warings
  * fixed refactoring bugs

  * added move optimizations

  * removed unused headers
  * removed unused dtors
  * use nullptr
  * removed unused vars