Added Send and Sync to NearestNeighbourIndex box.

[Christoff-Zulzi]

LinearSearch, KdTree and BallTree are all Send + Sync. Allows for async use of the models.

[codecov-commenter]

Codecov Report

Merging #176 (a11b682) into master (870107f) will increase coverage by 0.07%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   55.37%   55.44%   +0.07%     
==========================================
  Files          96       96              
  Lines        8918     8921       +3     
==========================================
+ Hits         4938     4946       +8     
+ Misses       3980     3975       -5     

Impacted Files	Coverage Δ
algorithms/linfa-nn/src/lib.rs	57.14% <ø> (ø)
algorithms/linfa-nn/src/linear.rs	43.75% <0.00%> (-1.71%)	⬇️
algorithms/linfa-nn/src/balltree.rs	54.33% <100.00%> (ø)
algorithms/linfa-nn/src/kdtree.rs	51.28% <100.00%> (+1.28%)	⬆️
...lgorithms/linfa-clustering/src/optics/algorithm.rs	48.53% <0.00%> (-0.29%)	⬇️
algorithms/linfa-nn/tests/nn.rs	78.04% <0.00%> (ø)
algorithms/linfa-kernel/src/lib.rs	60.21% <0.00%> (ø)
algorithms/linfa-logistic/src/lib.rs	69.55% <0.00%> (ø)
algorithms/linfa-pls/src/pls_generic.rs	69.64% <0.00%> (ø)
... and 6 more

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[YuhanLiin]

The type Box<dyn 'a + Send + Sync + NearestNeighbourIndex<F>> is getting a bit long. Can you replace it with an internal type alias?

[YuhanLiin]

Rebase this branch on top of the current master to get rid of the CI failure

[YuhanLiin]

Closed by #235