Improve type inference, add FBP examples

[codecov[bot]]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (05be028) compared to base (e090e51). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #21 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 15 16 +1 Lines 451 516 +65 ========================================= + Hits 451 516 +65 ``` | [Impacted Files](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler) | Coverage Δ | | |---|---|---| | [src/fbp/window.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicC93aW5kb3cuamw=) | `100.00% <ø> (ø)` | | | [src/Sinograms.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL1Npbm9ncmFtcy5qbA==) | `100.00% <100.00%> (ø)` | | | [src/fbp-par.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicC1wYXIuamw=) | `100.00% <100.00%> (ø)` | | | [src/fbp2/back-fan.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/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% <100.00%> (ø)` | | | [src/fbp2/back-par.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvYmFjay1wYXIuamw=) | `100.00% <100.00%> (ø)` | | | [src/fbp2/fbp.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvZmJwLmps) | `100.00% <100.00%> (ø)` | | | [src/fbp2/plan.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvcGxhbi5qbA==) | `100.00% <100.00%> (ø)` | | | [src/fbp2/sino-geom.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvc2luby1nZW9tLmps) | `100.00% <100.00%> (ø)` | | | [src/fbp2/sino-plot.jl](https://codecov.io/gh/JeffFessler/Sinograms.jl/pull/21/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Jeff+Fessler#diff-c3JjL2ZicDIvc2luby1wbG90Lmps) | `100.00% <100.00%> (ø)` | | 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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