pollin: Linearly dependent polarization

This multiplicative model applies a polarization with a linear dependence on energy. The factor applied depends on which Stokes parameter the spectrum is for. The Stokes parameter is determined using an XFLTnnnn keyword with values ``Stokes:0'', ``Stokes:1'', or ``Stokes:2'', for I, Q, U, respectively.

The fraction and angle are determined by

$$A(E) = {\tt A1} + (E-1.0) \times {\tt Aslope}$$

$$\psi(E) = {\tt psi1} + (E-1.0) \times {\tt psislope}$$

The parameters are :

par1	A1, the polarization fraction at 1 keV
par2	Aslope, the polarization fraction slope
par3	psi1, the polarization angle at 1 keV (degrees)
par4	psislope, the polarization angle slope