.. with grateful thanks to Greg Hamerly
A fast kmeans algorithm described here:
https://epubs.siam.org/doi/10.1137/1.9781611972801.12
The source repository is also here:
https://github.com/ghamerly/fast-kmeans
NOTE:
The original source has been included largely as-is with
a view to writing a wrapper around it using Qt semantics
for use in GoldenCheetah (e.g. via datafilter)
The original source included multiple kmeans algorithms
we have only kept the `fast' Hamerly variant.
