helix.jl and fbp_helix_stack.jl

[hnanda22]

Initial commit, commenting and styling still left to do

[codecov[bot]]

Codecov Report

Merging #15 (d053c3f) into main (442e7e0) will decrease coverage by 0.45%. The diff coverage is n/a.

:exclamation: Current head d053c3f differs from pull request most recent head 959f42d. Consider uploading reports for the commit 959f42d to get more accurate results

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
- Coverage   29.97%   29.51%   -0.46%     
==========================================
  Files           9        9              
  Lines         337      332       -5     
==========================================
- Hits          101       98       -3     
+ Misses        236      234       -2     

Impacted Files	Coverage Δ
src/fbp2.jl	18.55% <0.00%> (-0.84%)	:arrow_down:
src/fbp2_back.jl	33.78% <0.00%> (-0.43%)	:arrow_down:
src/zwart_powell.jl	100.00% <0.00%> (ø)
src/fbp2_back_fan.jl	0.00% <0.00%> (ø)

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[codecov[bot]]

Codecov Report

Base: 99.63% // Head: 99.63% // No change to project coverage :thumbsup:

Coverage data is based on head (958c3c7) compared to base (e294313). Patch has no changes to coverable lines.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15 +/- ## ======================================= Coverage 99.63% 99.63% ======================================= Files 24 24 Lines 828 828 ======================================= Hits 825 825 Misses 3 3 ``` 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=None). 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=None)

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