Avoid panic when a coordinate is NaN

[msk]

No longer panics when a coordinate is NaN; the distance from a point with
NaN in its coordinate and another point is considered greater than the
distance between any two points without NaN in their coordinates.

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 0.27%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   92.36%   92.63%   +0.27%     
==========================================
  Files           3        3              
  Lines         419      421       +2     
==========================================
+ Hits          387      390       +3     
+ Misses         32       31       -1     

Impacted Files	Coverage Δ
src/ball_tree.rs	93.71% <83.33%> (+0.03%)	⬆️
src/vantage_point_tree.rs	87.17% <100.00%> (+1.28%)	⬆️

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