[ENH] Level capabilities refactor for hashed level

[adam2392]

Towards: #19

The hashed.hpp file implemented its own iteration_helper class, whereas the Coiterate API relies on each level having iteration helper within a level_capabilities namespace.

This PR attempts to refactor the iteration_helper part of hashed into the coordinate_iterate.hpp file.

Bottlenecks

For some reason I get compilation errors that iter_helper is not found, even tho it is now part of the hashed level through the level_capabilities namespace:

/Users/adam2392/Documents/xsparse/test/source/hashed_test.cpp:31:34: error: no member named 'iter_helper' in 'xsparse::levels::hashed<std::tuple<>, unsigned long, unsigned long, xsparse::util::container_traits<std::vector, std::set, std::unordered_map>, xsparse::level_properties<false, false, false, false, false>>'
    for (auto const [i2, p2] : h.iter_helper(ZERO))
                               ~ ^

and also

/Users/adam2392/Documents/xsparse/include/xsparse/util/base_traits.hpp:34:9: error: static_assert failed due to requirement 'std::is_convertible_v<xsparse::util::container_traits<std::vector, std::set, std::unordered_map>, unsigned long>' "`PK` must be convertible to uintptr_t."
        static_assert(std::is_convertible_v<PK, uintptr_t>,
        ^             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Notes from July 11th, 2023:

• Change Levels::LevelCapabilities::iteration_helper to Levels::iteration_helper to remove requirement of namespace LevelCapabilities in Coiteration
• Add maybe-unused parameter I to the initialization of hashed level iteration_helper

[bharath2438]

Hey @adam2392 @hameerabbasi,
I was trying to find a better way of accessing LevelCapabilities of hashed and found that using LevelCapabilities = typename xsparse::levels::hashed<std::tuple<LowerLevels...>, IK, PK, ContainerTraits, _LevelProperties>; in hashed.hpp without having to move the iteration logic to coordinate_iterate.hpp does work.
Another reason why we should avoid moving the code is because https://github.com/hameerabbasi/xsparse/blob/9f47a2cfc37ac9a2bf7197f6489df87bb2bddb83/include/xsparse/levels/hashed.hpp#LL132C14-L132C14 here, we need m_crd which is in the scope of hashed level.

Fixing this and adding the [[maybe_unused]] I parameter should make hashed consistent with other levels and (hopefully) allow coiteration with other levels.

[adam2392]

Hey @adam2392 @hameerabbasi, I was trying to find a better way of accessing LevelCapabilities of hashed and found that using LevelCapabilities = typename xsparse::levels::hashed<std::tuple<LowerLevels...>, IK, PK, ContainerTraits, _LevelProperties>; in hashed.hpp without having to move the iteration logic to coordinate_iterate.hpp does work. Another reason why we should avoid moving the code is because https://github.com/hameerabbasi/xsparse/blob/9f47a2cfc37ac9a2bf7197f6489df87bb2bddb83/include/xsparse/levels/hashed.hpp#LL132C14-L132C14 here, we need m_crd which is in the scope of hashed level.

Fixing this and adding the [[maybe_unused]] I parameter should make hashed consistent with other levels and (hopefully) allow coiteration with other levels.

Hi @bharath2438 I've opened up #25 to supersede this PR and added that logic. Lmk if that is what you had in mind?