Implement HPX::in_parallel

[msimberg]

Fixes #5744. This implements the weaker form of in_parallel from https://kokkos.github.io/kokkos-core-wiki/API/core/execution_spaces.html?highlight=in_parallel#functionality:

Note: as currently implemented, there is no guarantee that true means the caller is necessarily executing as part of a pattern on the particular instance ex; just some instance of Ex. This may be strengthened in the future.

i.e. this does not use the ExecutionSpace argument to in_parallel. I can strengthen this if useful in the future.

I couldn't find any existing tests (except for

kokkos/core/unit_test/incremental/Test01_execspace.hpp

Lines 65 to 66 in 62d2b6c

	int in_parallel = ExecSpace::in_parallel();
	ASSERT_FALSE(in_parallel);

which doesn't test much) so I added a test specific for the HPX backend. Let me know if this test would be possible to use already now for all backends.

[masterleinad]

Looks OK to me. It seems we could extend the tests added to all other backends (except for Serial) but I'm fine with doing that in another pull request.

[msimberg]

Do you know what happened on continuous-integration/jenkin-ci/pr? Anything I should worry about?

[dalg24]

Do you know what happened on continuous-integration/jenkin-ci/pr? Anything I should worry about?

I do not but it looks like the clang-format test passed so it is probably ok.

[msimberg]

Any further comments on this?

[dalg24]

No. Looks good. Thank you Mikael.