Fix snr2sigma

[JeffFessler]

Because randn for complex data in Julia already adjusts for sqrt(2).

[codecov]

Codecov Report

Merging #115 (301fd8b) into main (4b94009) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #115      +/-   ##
==========================================
- Coverage   99.95%   99.95%   -0.01%     
==========================================
  Files          45       45              
  Lines        2262     2192      -70     
==========================================
- Hits         2261     2191      -70     
  Misses          1        1              

Impacted Files	Coverage Δ
src/mri/coil_compress.jl	100.00% <100.00%> (ø)
src/algorithm/general/pogm_restart.jl	98.82% <0.00%> (-0.02%)	⬇️
src/io/ir_dump.jl	100.00% <0.00%> (ø)
src/nufft/dtft.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)
src/fbp/rect_sino.jl	100.00% <0.00%> (ø)
src/fbp/sino_plot.jl	100.00% <0.00%> (ø)
... and 18 more

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