Update OpenMP Kernels

[radelja]

Overview

This PR updates the OpenMP kernels to address an issue with the gather kernel and aligns them closer to their v1.1 implementations. As mentioned in #189, there is still a gap in performance between the current scatter, multiscatter, and sg OpenMP kernels on certain platforms.

✨ Change Description/Rationale

• Remove duplicate gather operation in the gather OpenMP kernel
• Align the OpenMP kernels closer to the OpenMP kernels in v1.1
• Use the dense_perthread buffers in the scatter and multiscatter OpenMP kernels

👀 Reviewer Checklist

• [ ] All GitHub actions and runners have passed if applicable
• [x] Commits are clean and relevant

✅ PR Checklist

• [x] Remove or update the template boilerplate text
• [x] Commits are relevant and combined where appropriate
• [x] Rebase off spatter-devel
• [x] Reviewers Requested
• [ ] Projects associated
• [x] Commits mention issue and/or PR numbers at the bottom of the message
• [x] Relevant issues are linked into the PR
• [x] TODOs are completed
• [ ] Reviewer checklist is updated

🚀 TODOs

• [ ] No additional TODOs for this PR

📌 Future Work

• Performance alignment of the scatter, multiscatter, and sg kernels on certain platforms (Cascade Lake, Ice Lake, Sandy Bridge...)

[plavin]

Looks good and runs fine on my machine. Once Jeff's comment about atomics is resolved that we can merge