polrot: Rotate polarization

This mixing model rotates the polarization assuming three spectra, one each for the I, Q, and U Stokes parameters. The I spectrum is unchanged, $Q_{new} = Q\times cos(2\theta) - U\times sin(2\theta)$, and $U_{new} = Q\times sin(2\theta) + U\times cos(2\theta)$.

Which spectrum is from which Stokes parameter is determined using an XFLTnnnn keyword with values ``Stokes:0'', ``Stokes:1'', or ``Stokes:2'', for I, Q, U, respectively.

The parameters are :

par1	theta, the polarization rotation angle (degrees)