Version 2.1.1

• Fixes and internal improvements:
  • Where available, CMake dependency downloading changed from ExternalProject to FetchContent.
  • cluster works with inactive molecules (still not dynamically updated, though)
  • improved memory efficiency for FunctorPotential
  • developer travis/osx build: mpich back to openmpi 1.6 due to abort trap 6 error (compiler mismatch)
  • cmake build compatible with conda-forge (circle/ci, travis)
  • fix compiler warning on GCC7
  • MPI destruction / MPI_Finalize() order fix for more stable MPI runs