Use tuples

[JeffFessler]

adresses #49

[codecov]

Codecov Report

Merging #55 into master will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   98.81%   98.86%   +0.04%     
==========================================
  Files          48       48              
  Lines        2107     2107              
==========================================
+ Hits         2082     2083       +1     
+ Misses         25       24       -1     

Impacted Files	Coverage Δ
src/utility/downsample.jl	100.00% <ø> (ø)
src/fbp/ellipse_sino.jl	100.00% <100.00%> (ø)
src/fbp/image_geom.jl	100.00% <100.00%> (ø)
src/fbp/rect_sino.jl	100.00% <100.00%> (ø)
src/mri/mri_trajectory.jl	100.00% <100.00%> (ø)
src/mri/mri_objects.jl	100.00% <0.00%> (+0.61%)	⬆️

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