Threaded version of pixel-driven back-projection

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 99.80% // Decreases project coverage by -0.19% :warning:

Coverage data is based on head (66bcf87) compared to base (30d3d6e). Patch coverage: 96.87% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #22 +/- ## =========================================== - Coverage 100.00% 99.80% -0.20% =========================================== Files 16 16 Lines 516 519 +3 =========================================== + Hits 516 518 +2 - Misses 0 1 +1 ``` | [Impacted Files](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/22?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler) | Coverage Δ | | |---|---|---| | [src/fbp2/back-fan.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/22/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvYmFjay1mYW4uamw=) | `100.00% <ø> (ø)` | | | [src/fbp2/back-par.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/22/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvYmFjay1wYXIuamw=) | `97.36% <96.87%> (-2.64%)` | :arrow_down: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler)

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