Allow named index-rank pairs in parallel tests

[masterleinad]

Split from #737, addressing #737 (comment).

Currently, we are using Kokkos::pair<int, int> for all distributed tests to denote index and rank for found matches. This is error-prone since they can't easily be distinguished.

[aprokop]

There's an awful lot going on in this PR, and I'm very confused. I thought the only thing that needed to be done was to introduce something like

struct PairIndexRank {
  int index;
  int rank;
};

[masterleinad]

Everything in this pull request is a consequence/requirement for changing

using PairIndexRank = Kokkos::pair<int, int>;
using TupleIndexRankDistance = Kokkos::pair<Kokkos::pair<int, int>, float>;

to

struct PairIndexRank
{
  int index;
  int rank;
};
using TupleIndexRankDistance = Kokkos::pair<PairIndexRank, float>;

[aprokop]

Everything in this pull request is a consequence/requirement for changing

I'm missing something. Why would you need to change the order of template arguments, for example, for this change?

[masterleinad]

I'm missing something. Why would you need to change the order of template arguments, for example, for this change?

I need to specify all template arguments and if I revert the order I can at least omit the template parameters for Indexable and Predicates (at least for the non-parallel RTree; for the parallel Rtree the compiler picks the wrong overload when omitting those). So OK, this was a small drive-by improvement and not strictly necessary.

[aprokop]

I need to specify all template arguments and if I revert the order

Again, why? For example, in Search_UnitTestHelpers.hpp you don't need ValueType template parameter, as you can do the same conditional as was done before, except for replacing Kokkos::Pair with a custom struct. I really don't understand why replacing Kokkos::pair<int,int> with IndexPair in some places requires these changes.