Allow kmeans with no iterations

[thempel]

• more consistent with api (user expects no iterations if max_iter=0)
• simpler to extract kmeans++ center initialization, in particular for pyemma

[clonker]

:D

[codecov]

Codecov Report

Merging #189 (c79f070) into main (6c0d297) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #189   +/-   ##
=======================================
  Coverage   93.50%   93.50%           
=======================================
  Files         131      131           
  Lines       10468    10468           
=======================================
  Hits         9788     9788           
  Misses        680      680           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6c0d297...c79f070. Read the comment docs.