Add weight factor for layouts
For 2D interference functions (2DLattice, 2DParacrystal, HardDisk) the particle density is calculated directly from lattice parameters and thus cannot be adjusted independently.
setTotalParticleSurfaceDensity
has no effect because ParticleLayout::totalParticleSurfaceDensity
checks whether the separately set particle density can be applied.
But there is still a need for independent weighting of layouts with 2D interference functions. When setting parameter distributions, we create a number of layouts but their total weight must be 1.
Now the scattering intensity is proportional to the number of distribution samples.