This commit adds the flag VTKm_ENABLE_GPU_MPI which when enable it
will use GPU AWARE MPI.
- This will only work with GPUs and MPI implementation that supports GPU
AWARE MPI calls.
- Enabling VTKm_ENABLE_GPU_MPI without MPI/GPU support might results in
errors when running VTK-m with DIY/MPI.
- Only the following tests can run with this feature if enabled:
- UnitTestSerializationDataSet
- UnitTestSerializationArrayHandle
Often when a user gives memory to an `ArrayHandle`, she wants data to be
written into the memory given to be used elsewhere. Previously, the
`Buffer` objects would delete the given buffer as soon as a write buffer
was created elsewhere. That was a problem if a user wants VTK-m to write
results right into a given buffer.
Instead, when a user provides memory, "pin" that memory so that the
`ArrayHandle` never deletes it.
The buffer class encapsulates the movement of raw C arrays between
host and devices.
The `Buffer` class itself is not associated with any device. Instead,
`Buffer` is used in conjunction with a new templated class named
`DeviceAdapterMemoryManager` that can allocate data on a given
device and transfer data as necessary. `DeviceAdapterMemoryManager`
will eventually replace the more complicated device adapter classes
that manage data on a device.
The code in `DeviceAdapterMemoryManager` is actually enclosed in
virtual methods. This allows us to limit the number of classes that
need to be compiled for a device. Rather, the implementation of
`DeviceAdapterMemoryManager` is compiled once with whatever compiler
is necessary, and then the `RuntimeDeviceInformation` is used to
get the correct object instance.
