Implement hierarchical clustering

[Mec-iS]

Draft for #11

For now: just a bare idea about what we need to implement with basic orientative docstring and simple test.

cc: @titoeb @danieljrmay

[codecov-commenter]

Codecov Report

Merging #146 (3fe9169) into development (d305406) will increase coverage by 0.06%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #146      +/-   ##
===============================================
+ Coverage        83.90%   83.97%   +0.06%     
===============================================
  Files               81       81              
  Lines             8724     8724              
===============================================
+ Hits              7320     7326       +6     
+ Misses            1404     1398       -6     

Impacted Files	Coverage Δ
src/algorithm/neighbour/cover_tree.rs	84.29% <0.00%> (-0.42%)	⬇️
src/optimization/first_order/lbfgs.rs	94.44% <0.00%> (+1.58%)	⬆️
src/optimization/line_search.rs	92.00% <0.00%> (+10.00%)	⬆️

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