Fixes for compiling with libcxx
- execution policies have deleted copy constructor in libcxx
- pass by universal ref and forward if a template parameter
- pass by const-ref if not a template parameter
- add
DUNE_PDELAB_USE_BOOST_ATOMIC_REF
option- std::atomic_ref is not yet implemented in libstdcxx
- add missing includes (sstream for <<, numeric for std::partial_sum)