Update license field following SPDX 2.1 license expression standard

[frisoft]

The new recommendation is to follow the SPDX 2.1 standard. This allows automatic license verification software to work properly. Reference: https://doc.rust-lang.org/cargo/reference/manifest.html#the-license-and-license-file-fields

[codecov-commenter]

Codecov Report

Attention: 67 lines in your changes are missing coverage. Please review.

Comparison is base (4adece5) 37.08% compared to head (d960fc0) 36.66%.
Report is 4 commits behind head on master.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
- Coverage   37.08%   36.66%   -0.43%     
==========================================
  Files          94       94              
  Lines        6196     6240      +44     
==========================================
- Hits         2298     2288      -10     
- Misses       3898     3952      +54     

Files	Coverage Δ
algorithms/linfa-ftrl/src/hyperparams.rs	15.38% <ø> (ø)
algorithms/linfa-ftrl/src/lib.rs	16.66% <ø> (+8.33%)	⬆️
algorithms/linfa-logistic/src/float.rs	0.00% <ø> (ø)
algorithms/linfa-logistic/src/hyperparams.rs	32.35% <ø> (-5.89%)	⬇️
src/dataset/mod.rs	33.33% <ø> (+3.70%)	⬆️
src/metrics_classification.rs	38.36% <ø> (+0.78%)	⬆️
algorithms/linfa-logistic/src/lib.rs	45.62% <47.82%> (-1.83%)	⬇️
algorithms/linfa-logistic/src/argmin_param.rs	10.95% <1.78%> (-33.49%)	⬇️

... and 65 files with indirect coverage changes

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