Improvements to DataSet and Vector to allow us to copy datasets and remove
columns from the vector.

A new filter to allow us to extract any arbitrary column as the label (useful
when processing output files from Weka's clustering algorithms).
Added code to let us append the dataset to an arff file.  This involves letting
the DataSetWriter append (instead of overwrite) and also letting the
DataSetWriter accept an array of classnames.
Changes to support dimensionality reduction using PCA and Insignificant
component analysis.  These classes now accept a varianceToKeep parameter, which
will then determine (by examining the eigenvalues) how many components to keep
to capture the specified amount of variance.  This value is a ceiling...the
classes may keep less but they will never keep more.