Added clean() method

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10631 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Updated for the next release.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10632 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Updated after fix to AbstractClassifier for date class attributes

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10633 e0a1b77d-ad91-4216-81b1-defd5f83fa92