Added a "<none>" option to the downstream drop-down boxes so that it is no longer necessary to have two connected downstream steps.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10638 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Added a "<none>" option to the downstream drop-down boxes so that it is no longer necessary to have two connected downstream steps.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10639 e0a1b77d-ad91-4216-81b1-defd5f83fa92