Tpetra: Add optional 'atomic' argument to sumIntoLocalValue(s)

Created by: mhoemmen

@trilinos/tpetra

For Tpetra::MultiVector, Tpetra::Vector, and Tpetra::CrsMatrix, add an optional bool atomic argument to sumIntoLocalValue resp. sumIntoLocalValues. If true, use Kokkos::atomic_add to do the +=; else, use +=. The default value of atomic should depend on the class' execution space: it should be false if and only if the execution space is Kokkos::Serial (so that existing MPI-only users don't have to pay for atomic updates).