'The dark energy equation of state.
Parameters
z : array-like
Input redshifts.
Returns
w : ndarray, or float if input scalar
The dark energy equation of state
Notes
The dark energy equation of state is defined as
:math:`w(z) = P(z)/\rho(z)`, where :math:`P(z)` is the
pressure at redshift z and :math:`\rho(z)` is the density
at redshift z, both in units where c=1.
This must be overridden by subclasses.'

@abstractmethod
def w(self, z):

    raise NotImplementedError(u'w(z)    is    not    implemented')
