Andrey Prokopenko

Getting some errors when building inside the container ``` {standard input}: Assembler messages: {standard input}:21745563: Warning: end of file not at end of a line; newline inserted {standard input}:21746825: Error:...

@dalg24 Could you please take a look at this PR?

Moved 'Concepts' namespace inside `Details` to hide them from users.

Here are what the errors look like for the `tstCompileOnlyAccessTraits.cpp`: In LLVM: ``` /test/tstCompileOnlyAccessTraits.cpp:137:7: error: static assertion failed 137 | ArborX::Details::Concepts::AccessTraits); | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ /test/tstCompileOnlyAccessTraits.cpp:137:7: note: because 'NoAccessTraitsSpecialization' does not satisfy...

It seems that the details of notes seem to depend on whether `static_assert` does any computations. For example, `static_assert(ArborX::Details::Concepts::AccessTraits);` fails with ``` /tstCompileOnlyAccessTraits.cpp.o -MF test/CMakeFiles/ArborX_Test_CompileOnly.exe.dir/tstCompileOnlyAccessTraits.cpp.o.d -o test/CMakeFiles/ArborX_Test_CompileOnly.exe.dir/tstCompileOnlyAccessTraits.cpp.o -c /test/tstCompileOnlyAccessTraits.cpp /test/tstCompileOnlyAccessTraits.cpp:137:7:...

@dalg24 I added couple commits: 1) Added a more thorough testing for `Predicates` concept, which also made `is_valid_predicate_tag` a concept 2) Added `complete_type` concept which checks whether specialization exists. I'm...

Rebased on #1265 and squashed.

> Please share perf test results Don't have any. This is just an observation not related to any particular problem I've run.

> Since we had to reimplement `Iota` several times, it makes sense to move it to ArborX. I'm not opposed to moving `Iota` to `ArborX::`. Why did you need to...

> It would be nice to run a benchmark to compare the performance difference but otherwise I like it. I did do that at some point (see [here](https://github.com/aprokop/ArborX/blob/apiv1/benchmarks/develop/develop.cpp)). Would need...