Set the observer angle relative to the field.

        **Call signature**

        *theta_rad*
          The angle between the ray path and the local magnetic field,
          in radians.
        Returns
          *self* for convenience in chaining.

def set_obs_angle(self, theta_rad):
        """Set the observer angle relative to the field.

        **Call signature**

        *theta_rad*
          The angle between the ray path and the local magnetic field,
          in radians.
        Returns
          *self* for convenience in chaining.
        """
        self.in_vals[IN_VAL_THETA] = theta_rad * 180 / np.pi # rad => deg
        return self