Further small improvements in runtime, primarily by introducing an UnsafeInstance inner subclass that extends DenseInstance and has an unsafe setValue() method.

git-svn-id: https://svn.cms.waikato.ac.nz/svn/weka/trunk@10399 e0a1b77d-ad91-4216-81b1-defd5f83fa92
Merge remote-tracking branch 'upstream'