Replace undef with similar

[JeffFessler]

For better GPU / CUDA compatibility.
Also updating tabs to 4space and updating type interference checks.

[codecov]

Codecov Report

Merging #113 (7f749d6) into main (1be5a79) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #113   +/-   ##
=======================================
  Coverage   99.95%   99.95%           
=======================================
  Files          45       45           
  Lines        2194     2262   +68     
=======================================
+ Hits         2193     2261   +68     
  Misses          1        1           

Impacted Files	Coverage Δ
src/nufft/dtft.jl	100.00% <100.00%> (ø)
src/system/Afft.jl	100.00% <100.00%> (ø)
src/utility/downsample.jl	100.00% <100.00%> (ø)
src/utility/map_many.jl	100.00% <100.00%> (ø)
src/io/ir_dump.jl	100.00% <0.00%> (ø)
src/fbp/rect_im.jl	100.00% <0.00%> (ø)
src/mri/exp_mult.jl	100.00% <0.00%> (ø)
src/utility/mask.jl	100.00% <0.00%> (ø)
src/fbp/cuboid_im.jl	100.00% <0.00%> (ø)
... and 19 more

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