Template lambda for scalar calls

[Yurlungur]

PR Summary

The work @gopsub is doing requires passing in many mesh-sized arrays into the lambda parameter. Currently on a per-cell basis, this requires packing those arrays into local contiguous arrays for lambdas. This can be cumbersome and expensive, especially for EOSs where that data is unneeded and is just a no-op.

This PR adds indexers/accessors to the lambda parameter for scalar calls, meaning data can now be passed in in a layout agnostic way. This should also reduce pain points in phoebus/riot, where the lambda arrays are used regularly and currently require contiguous accesses.

PR Checklist

• Adds a test for any bugs fixed. Adds tests for new features.
• Format your changes by using the make format command after configuring with cmake.
• Document any new features, update documentation for changes made.
• Make sure the copyright notice on any files you modified is up to date.
• After creating a pull request, note it in the CHANGELOG.md file.
• LANL employees: make sure tests pass both on the github CI and on the Darwin CI

If preparing for a new release, in addition please check the following:

• Update the version in cmake.
• Move the changes in the CHANGELOG.md file under a new header for the new release, and reset the categories.

[Yurlungur]

This is now ready for review. @dholladay00 @gopsub @jhp-lanl

[Yurlungur]

Ping @jhp-lanl @dholladay00 reminder to review.

[jhp-lanl]

Looks good to me. I appreciate the is_nullptr function. I think it's very clear now what's going on and if I want to wade into the template nonsense I can look at the variadic_utils.hpp file 🙂

[Yurlungur]

@dholladay00 I think I addressed all your comments. Can you re-review and approve if you're happy?

[dholladay00]

@dholladay00 I think I addressed all your comments. Can you re-review and approve if you're happy?

Is this passing gitlab CI @Yurlungur ?

[Yurlungur]

@dholladay00 I think I addressed all your comments. Can you re-review and approve if you're happy?

Is this passing gitlab CI @Yurlungur ?

Running. Won't merge until it passes.

[Yurlungur]

Addressed @mauneyc-LANL 's comments. As soon as tests pass on re-git I will merge.

[Yurlungur]

Commenting here for posterity that when we switch to C++17 we will be able to do the following, which will allow us to properly handle passing in nullptr:

template <typename T>
struct remove_cvref {
  typedef std::remove_cv_t<std::remove_reference_t<T>> type;
};
template <typename T>
using remove_cvref_t = typename remove_cvref<T>::type;

template<typename T>
constexpr inline bool is_null(T &&t) {
    if constexpr(std::is_pointer<remove_cvref_t<T>>::value) {
        return std::forward<T>(t) == nullptr;
    } else {
        return false;
    }
}

#define SG_DO_IF_NOT_NULL(lambda) \
  if constexpr (!std::is_null_pointer<remove_cvref_t<decltype(lambda)>>::value) \
    if (!is_null(lambda)) 

template<typename T>
void DoSomething(T &&t) {
    SG_DO_IF_NOT_NULL(t) {
        std:: cout << t[0] << std::endl;
    }
}

int main() {

    auto q = nullptr;
    std::vector<Real> v = {1, 2, 3, 4};
    DoSomething(v);
    DoSomething(q);

    return 0;
}

[Yurlungur]

Tests pass on re-git. Merging.