Support specular intensity for polarized GISANS

The choice simulation.options().setIncludeSpecular(True) is currently incompatible with polarized GISANS (simulation with polarization analysis or sample with magnetic material). Computation will fail in file Sim/Contrib/GISASSpecularContribution.cpp, function compute::gisasSpecularContribution, after attempt to cast fluxIn into a SpecularFlux so that flux->getScalarR() can be called.

This replaces #75 (closed).

Test script:

#!/usr/bin/env python3
"""
Minimum example of crash when using a magnetic material together with setIncludeSpecular(True).

Based on MagneticSpheres.py example.
"""
import bornagain as ba
from bornagain import ba_plot as bp, deg, nm, R3

def get_sample():
    """
    Returns a sample with magnetic spheres in the substrate.
    """

    # Define materials
    magnetic_field = R3(0, 0, 1e7)
    material_Layer = ba.RefractiveMaterial("Layer", 2e-05*1e-3, 4e-07*1e-3,
                                               magnetic_field*1e-3)
    material_Particle = ba.RefractiveMaterial("Particle", 2e-05, 4e-07,
                                               magnetic_field)
    material_Substrate = ba.RefractiveMaterial("Substrate", 7e-06,
                                                1.8e-07)
    material_Vacuum = ba.RefractiveMaterial("Vacuum", 0, 0)

    # Define form factors
    ff = ba.Sphere(5*nm)

    # Define particles
    particle = ba.Particle(material_Particle, ff)
    particle.translate(0, 0, -10*nm)

    # Define particle layouts
    layout = ba.ParticleLayout(particle)
    layout.setTotalParticleSurfaceDensity(0.01)

    # Define layers
    layer_1 = ba.Layer(material_Vacuum)
    layer_2 = ba.Layer(material_Layer)
    layer_2.addLayout(layout)
    layer_3 = ba.Layer(material_Substrate)

    # Define sample
    sample = ba.MultiLayer()
    sample.addLayer(layer_1)
    sample.addLayer(layer_2)
    sample.addLayer(layer_3)

    return sample


def get_simulation(sample):
    beam = ba.Beam(1e12, 0.1*nm, 0.5*deg, 0)
    beam_polarization = R3(0, 1, 0)
    beam.setPolarization(beam_polarization)
    detector = ba.SphericalDetector(200, 6*deg, 0, 3*deg)
    detector.setAnalyzer(R3(0, 1, 0), 1, 0.5)
    simulation = ba.ScatteringSimulation(beam, sample, detector)
    simulation.options().setIncludeSpecular(True) # simulation runs normally without this line
    return simulation


if __name__ == '__main__':
    bp.parse_args()
    sample = get_sample()
    simulation = get_simulation(sample)
    result = simulation.simulate()
    bp.plot_simulation_result(result)