Feature proposal: add data read/write routines

[aprokop]

Purpose

Facilitate easier examination of user data by allowing to save primitives and queries and subsequently share with us. Immediate use cases: NimbleSM (to study OpenMP/MPI scaling), CabanaMD (to play with the problem).

API

Something along the lines of

namespace ArborX {

template<typename Data>
void saveData(std::string const& filename, Data const& data, bool binary = false);

template<typename Data>
void saveData(std::string const& filename, MPI_Comm comm, Data const& data, bool binary = false);

template<typename MemorySpace>
auto loadData(std::string const& filename, bool binary = false);

template<typename MemorySpace>
auto loadData(std::string const& filename, MPI_Comm comm, bool binary = false);

}

The filenames automatically postfixed with .arborx. For distributed runs, they are postfixed with .<rank>.arborx.

Some ideas

We want to be able to save both primitives and predicates data.

For primitives, we save Kokkos::View<T*,MemorySpace> (either provided by a user, or constructed through AccessTraits). The metadata should include:

• [optional] little endian/big endian (if saving in binary mode)
• Size (number of primitives)
• [optional] Data type Point/Box (we could deduce it from the sizes of the file, though)
• Data

For predicates, we save Geometry (and k for the nearest). We can limit ourselves with Point and Box for the geometry for now, but think about providing a way to treat custom geometry as well in the future. In addition, possibly treat attachments. Callback will probably be harder.