labeler tests

Add tests for labeler; still need to test missing data, but that
feature isn't implemented yet.
remove exemplar class

This class was just a wrapper around `Instance`, and had no clear
purpose while also making the code unclear, so remove it.

Notice the strange required use of `toIntValue` in
`AnalogicalModelingOutput`. Weka uses ints in some places and doubles
in others for indexing attribute values. How annoying is that! This
may be a misunderstanding on my part, though.
fix analogical set printing and report tests

Sort the contents of the internal sets before stringification
in `AnalogicalSet` so that this output is easily testable. Fix
problems with accessing class strings, and change class/effect
maps to store the class names instead of their index doubles.

Update AnalogicalModelingOutput to print the AnalogicalSet object
instead of iterating the data and printing manually. Change it's test
to simply check that an AnalogicalSet was printed, since a test of
AnalogicalSet will probably check the details of the stringification.