-------------------
- added `class_index` parameter to `weka.core.converters.load_any_file`
and `weka.core.converters.Loader.load_file`, which allows specifying of
index while loading it (`first`, `second`, `third`, `last-2`, `last-1`,
`last` or 1-based index).
- added `append` and `clear` methods to `weka.filters.MultiFilter` and
`weka.classifiers.MultipleClassifiersCombiner` to make adding of
filters/classifiers easier.
- added `attribute_names()` method to `weka.core.dataset.Instances` class
- added `subset` method to `weka.core.dataset.Instances` class, which returns
a subset of columns and/or rows.