Bug fix: stratification now not used if class is numeric. Code cleanup. New parameter that allows look-ahead, and look-ahead in place by default.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10943 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Now implements multi-threading.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10944 e0a1b77d-ad91-4216-81b1-defd5f83fa92