Added a getMode() method to NominalStats

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10594 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Renamed the reset() method in Preconstructed to resetPreconstructed() in order to avoid a clash with reset() in SimpleFilter

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10595 e0a1b77d-ad91-4216-81b1-defd5f83fa92
getMode() now returns -1 if a nominal attribute is empty.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka@10596 e0a1b77d-ad91-4216-81b1-defd5f83fa92