Refactor KMeans

- change the way initial cluster centers are set
- add option to keep cluster centers fixed (1 pass...)
- use SGMatrix internally
- split up clustknb into smaller units
Drop unused variables
update news
drop useless SERIALIZABLE DUMMY
Fix FGM notebook
Do not use LabelsFactory in Python for apply's output, the cast is already
handled by SWIG code in Machine.i
Merge pull request #1806 from iglesias/develop

Fix FGM notebook
use %newobject on label factory functions to fix premature memory freeing
Remove LabelsFactory.to_structured in python modular example
NO need to use this method to do the cast since it is already handled
in Machine.i
Merge pull request #1807 from iglesias/develop

Remove LabelsFactory.to_structured in python modular example
Update bugfix NEWS for latest release
Merge pull request #1808 from iglesias/develop

Update bugfix NEWS for latest release