Add python example for constant background

Examples/python/simulation/ex05_BeamAndDetector/ConstantBackground.py

+"""
+Cylinder form factor in DWBA with constant background
+"""
+import bornagain as ba
+from bornagain import deg, angstrom, nm
+
+
+def get_sample():
+    """
+    Returns a sample with uncorrelated cylinders on a substrate.
+    """
+    # defining materials
+    m_ambience = ba.HomogeneousMaterial("Air", 0.0, 0.0)
+    m_substrate = ba.HomogeneousMaterial("Substrate", 6e-6, 2e-8)
+    m_particle = ba.HomogeneousMaterial("Particle", 6e-4, 2e-8)
+
+    # collection of particles
+    cylinder_ff = ba.FormFactorCylinder(5*nm, 5*nm)
+    cylinder = ba.Particle(m_particle, cylinder_ff)
+    particle_layout = ba.ParticleLayout()
+    particle_layout.addParticle(cylinder, 1.0)
+
+    # assembling the sample
+    air_layer = ba.Layer(m_ambience)
+    air_layer.addLayout(particle_layout)
+    substrate_layer = ba.Layer(m_substrate)
+
+    multi_layer = ba.MultiLayer()
+    multi_layer.addLayer(air_layer)
+    multi_layer.addLayer(substrate_layer)
+    return multi_layer
+
+
+def get_simulation():
+    """
+    Returns a GISAXS simulation with a constant backround.
+    """
+    simulation = ba.GISASSimulation()
+    simulation.setDetectorParameters(100, 0.0*deg, 2.0*deg,
+                                     100, 0.0*deg, 2.0*deg)
+    simulation.setBeamParameters(1.0*angstrom, 0.2*deg, 0.0*deg)
+    simulation.setBeamIntensity(1e6)
+    bg = ba.ConstantBackground(1e3)
+    simulation.setBackGround(bg)
+    return simulation
+
+
+def run_simulation():
+    """
+    Runs simulation and returns intensity map.
+    """
+    simulation = get_simulation()
+    simulation.setSample(get_sample())
+    print(simulation.parametersToString())
+    simulation.runSimulation()
+    return simulation.getIntensityData()
+
+
+if __name__ == '__main__':
+    result = run_simulation()
+    ba.plot_intensity_data(result)