Support the widely used: `flexclust::kcca()`

[m-muecke]

Quite useful would be the implementation: flexclust::kcca with initcent = kmeanspp for k-means++ initialisation as referenced in Arthur, David, and Sergei Vassilvitskii. "k-means++: The advantages of careful seeding." In Soda, vol. 7, pp. 1027-1035. 2007.