Buffer Particle

Adds an implementation detail providing the ability to take a single element of a Cabana::AoSoA as indicated by a given Cabana::Index and serialize all of the data members associated with that index into a byte stream represented by a Cabana::BufferParticle. In reverse, it also provides the ability to unpack the data members from a Cabana::BufferParticle into an individual Cabana::AoSoA element as indicated by a given Cabana::Index.

To support this a new function Cabana::Index::oneD() which creates a 1-dimensional index for a given Cabana::AoSoA element. This is useful for iterating through linear structures, such as a particle buffer.

This allows one to effectively do the following:

    // Data dimensions.
    const std::size_t dim_1 = 3;
    const std::size_t dim_2 = 2;
    const std::size_t dim_3 = 4;
    const std::size_t dim_4 = 3;

    // Declare member types.
    using T0 = float[dim_1][dim_2][dim_3];
    using T1 = int;
    using T2 = float[dim_1][dim_2][dim_3][dim_4];
    using T3 = double[dim_1];
    using T4 = double[dim_1][dim_2];

    // Declare data types.
    using DataTypes = Cabana::MemberDataTypes<T0,T1,T2,T3,T4>;

    // Declare the AoSoA type.
    using AoSoA_t = Cabana::AoSoA<DataTypes,Kokkos::CudaUVMSpace>;

    // Create an AoSoA.
    std::size_t num_data = 453;
    AoSoA_t aosoa( num_data );

    // Create a particle buffer. Effectively a giant byte stream because 
    // BufferParticle simply holds a char pointer.
    using BufferParticle = Cabana::Impl::BufferParticle<DataTypes>;
    Kokkos::View<BufferParticle*,Kokkos::CudaUVMSpace> buffer( "buffer", num_data );

    // Create an execution policy for packing/unpacking.
    Cabana::IndexRangePolicy<Kokkos::Cuda> policy( aosoa );

    // Pack the data into a buffer.
    auto pack_op = KOKKOS_LAMBDA( const Cabana::Index idx )
    {
        Cabana::Impl::pack( idx, aosoa, buffer(idx.oneD()) );
    };
    Cabana::parallel_for( policy, pack_op );

    // Make a new AoSoA.
    AoSoA_t aosoa_2( num_data );

    // Unpack the buffer into the new AoSoA.
    auto unpack_op = KOKKOS_LAMBDA( const Cabana::Index idx )
    {
        Cabana::Impl::unpack( idx, buffer(idx.oneD()), aosoa_2 );
    };
    Cabana::parallel_for( policy, unpack_op );