refactor: make AGNES algorithm closer to R

[targos]

BREAKING CHANGE:

• use the Lance-Williams algorithm to update cluster distances.
• add other methods that exist in R.
• remove ClusterLeaf class and use an isLeaf property instead.
• remove index array from clusters. Instead, an indexes() method has been added to compute it.
• add a size property to clusters that indicates the number of leaves below it.
• the default method is now 'complete'.
• DIANA has been removed from the package pending rewriting it.

[codecov]

Codecov Report

Merging #11 into master will decrease coverage by 14.63%.
The diff coverage is 71.42%.

@@             Coverage Diff             @@
##           master      #11       +/-   ##
===========================================
- Coverage   87.36%   72.72%   -14.64%     
===========================================
  Files           5        3        -2     
  Lines         269      176       -93     
  Branches       33       31        -2     
===========================================
- Hits          235      128      -107     
- Misses         33       47       +14     
  Partials        1        1

Impacted Files	Coverage Δ
src/agnes.js	63.33% <66.66%> (-12.53%)	⬇️
src/Cluster.js	91.48% <88.88%> (-3.52%)	⬇️

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