Use in, abandon 1.6, add gpu urls

[JeffFessler]

Testing 1.6 just increases carbon footprint for no good reason...

[codecov]

Codecov Report

Base: 98.92% // Head: 98.92% // No change to project coverage 👍

Coverage data is based on head (29ddf3c) compared to base (f5b496f).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #44   +/-   ##
=======================================
  Coverage   98.92%   98.92%           
=======================================
  Files          11       11           
  Lines         745      745           
=======================================
  Hits          737      737           
  Misses          8        8           

Impacted Files	Coverage Δ
src/psf-gauss.jl	100.00% <ø> (ø)
src/backproject.jl	93.93% <100.00%> (ø)
src/fft_convolve.jl	100.00% <100.00%> (ø)
src/helper.jl	100.00% <100.00%> (ø)
src/ml-os-em.jl	100.00% <100.00%> (ø)
src/plan-psf.jl	100.00% <100.00%> (ø)
src/plan-rotate.jl	100.00% <100.00%> (ø)
src/project.jl	95.08% <100.00%> (ø)
src/rotatez.jl	99.53% <100.00%> (ø)
src/spectplan.jl	100.00% <100.00%> (ø)

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