'Cartesian unit vectors in the direction of each component.
Given unit vectors :math:`\hat{e}_c` and scale factors :math:`f_c`,
a change in one component of :math:`\delta c` corresponds to a change
in representation of :math:`\delta c \times f_c \times \hat{e}_c`.
Returns
unit_vectors : dict of `CartesianRepresentation`
The keys are the component names.'

def unit_vectors(self):

    raise NotImplementedError(u'{}    has    not    implemented    unit    vectors'.format(type(self)))
