'Remove columns with constant values from the data set and return
the resulting data table.
Parameters
data : an input data set'

def __call__(self, data):

    oks = (bn.nanmin(data.X, axis=0) != bn.nanmax(data.X, axis=0))
    atts = [data.domain.attributes[i] for (i, ok) in enumerate(oks) if ok]
    domain = Orange.data.Domain(atts, data.domain.class_vars, data.domain.metas)
    return data.transform(domain)
