Allow default-initialized distributed tree

[aprokop]

This would, for example, allow storing DistributedTree in user classes for later initialization.

[aprokop]

All tests pass except for SYCL. Trying to address the latter in #1041.

[dalg24]

The empty tree state still does not feel right to me.
We did not spell out any precondition on the getComm() member. Should we be initializing to MPI_COMM_NULL or such?

[aprokop]

We did not spell out any precondition on the getComm() member. Should we be initializing to MPI_COMM_NULL or such?

Addressed. The preconditions to calling MPI_Comm_rank are still checked because MPI standard says that most calls cannot be called with MPI_COMM_NULL.

Whether getComm() should be a public member is an open question, and outside of the scope of this PR.

[dalg24]

Please do this instead

diff --git a/src/ArborX_DistributedTree.hpp b/src/ArborX_DistributedTree.hpp
index f31ea1b8..229b60cf 100644
--- a/src/ArborX_DistributedTree.hpp
+++ b/src/ArborX_DistributedTree.hpp
@@ -50,13 +50,7 @@ public:
   using bounding_volume_type = BoundingVolume;
   using value_type = typename BottomTree::value_type;

-  DistributedTreeBase()
-  {
-    _comm_ptr.reset([]() {
-      auto p = std::make_unique<MPI_Comm>(MPI_COMM_NULL);
-      return p.release();
-    }());
-  }
+  DistributedTreeBase() = default;

   template <typename ExecutionSpace, typename... Args>
   DistributedTreeBase(MPI_Comm comm, ExecutionSpace const &space,
@@ -115,7 +109,8 @@ protected:
 private:
   friend struct Details::DistributedTreeImpl;

-  std::shared_ptr<MPI_Comm> _comm_ptr;
+  std::shared_ptr<MPI_Comm> _comm_ptr{
+      std::make_unique<MPI_Comm>(MPI_COMM_NULL)};
   BottomTree _bottom_tree; // local
   TopTree _top_tree;       // replicated
   size_type _top_tree_size{0};