Constrains change in RV to be less than limit over time dt.

        Only works if ``dRV`` and ``Plong`` attributes are defined
        for population.

        :param limit:
            Radial velocity limit on trend.  Must be
            :class:`astropy.units.Quantity` object, or
            else interpreted as m/s.

        :param dt:
            Time baseline of RV observations.  Must be
            :class:`astropy.units.Quantity` object; else
            interpreted as days.

        :param distribution_skip:
            This is by default ``True``.  *To be honest, I'm not
            exactly sure why.  Might be important, might not
            (don't remember).*

        :param **kwargs:
            Additional keyword arguments passed to
            :func:`StarPopulation.apply_constraint`.

def apply_trend_constraint(self, limit, dt, distribution_skip=False,
                               **kwargs):
        """
        Constrains change in RV to be less than limit over time dt.

        Only works if ``dRV`` and ``Plong`` attributes are defined
        for population.

        :param limit:
            Radial velocity limit on trend.  Must be
            :class:`astropy.units.Quantity` object, or
            else interpreted as m/s.

        :param dt:
            Time baseline of RV observations.  Must be
            :class:`astropy.units.Quantity` object; else
            interpreted as days.

        :param distribution_skip:
            This is by default ``True``.  *To be honest, I'm not
            exactly sure why.  Might be important, might not
            (don't remember).*

        :param **kwargs:
            Additional keyword arguments passed to
            :func:`StarPopulation.apply_constraint`.

        """
        if type(limit) != Quantity:
            limit = limit * u.m/u.s
        if type(dt) != Quantity:
            dt = dt * u.day

        dRVs = np.absolute(self.dRV(dt))
        c1 = UpperLimit(dRVs, limit)
        c2 = LowerLimit(self.Plong, dt*4)

        self.apply_constraint(JointConstraintOr(c1,c2,name='RV monitoring',
                                                Ps=self.Plong,dRVs=dRVs),
                              distribution_skip=distribution_skip, **kwargs)