Accept an ndarray as a point

[msk]

This resolves #30.

[codecov]

Codecov Report

Merging #31 into master will increase coverage by 0.25%.
The diff coverage is 97.36%.

@@            Coverage Diff             @@
##           master      #31      +/-   ##
==========================================
+ Coverage   92.11%   92.36%   +0.25%     
==========================================
  Files           3        3              
  Lines         431      419      -12     
==========================================
- Hits          397      387      -10     
+ Misses         34       32       -2     

Impacted Files	Coverage Δ
src/ball_tree.rs	93.67% <96.66%> (+0.12%)	⬆️
src/distance.rs	100.00% <100.00%> (ø)
src/vantage_point_tree.rs	85.89% <100.00%> (+0.71%)	⬆️

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