Peter M. Groen / openbr

Change default label name from Subjet to Label (since label is a more general
term).
Use different default variable names for classification (label), regression
(regressor/regressand), and clustering (ClusterID)
Update some (far from all) transforms to accept arguments specifying their
input/output variables.
Update eval classification to optionally take target variable names as
arguments

Remove subject/label methods from the API, replace some methods with general
methods taking a property name as an argument.

This breaks quite a few things