CPU Improvements

[pravirkr]

• Buffer memory requirement reduced.
  • Need to use complicated channel indexing.
  • No easy way to visualize intermediate states with numpy.
  • Improved benchmarks.
• C++20
  • prefer span over raw pointers (safety)
• Merge for loops in execution
  • Keeping two loops for subband and dt is not good for parallelization.
• Move if else block outside for loop in execution
  • Keep separate lists for adding and copying.