Improve fld tests

[JeffFessler]

Improve fld test coverage.

[codecov]

Codecov Report

Merging #63 into master will increase coverage by 1.85%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
+ Coverage   97.76%   99.62%   +1.85%     
==========================================
  Files          48       48              
  Lines        2283     2120     -163     
==========================================
- Hits         2232     2112     -120     
+ Misses         51        8      -43     

Impacted Files	Coverage Δ
src/io/fld-write.jl	90.90% <91.66%> (+18.77%)	⬆️
src/io/fld-read.jl	97.82% <100.00%> (+15.68%)	⬆️
src/nufft/dtft.jl	100.00% <0.00%> (ø)
src/nufft/nufft.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/image_geom.jl	100.00% <0.00%> (ø)
src/mri/mri_objects.jl	100.00% <0.00%> (ø)
src/fbp/disk-phantom.jl	100.00% <0.00%> (ø)
... and 13 more

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