Useful 3D-background models

3D-models useful to describe non-factorazable (f(x,y,z)!=f(x)*f(y)*f(z)) background:

• PolyPos3D_pdf : positive (non-negative) polynomial in 3D
• PolyPos3Dsym_pdf : positive (on-negative) symmetric polynomial in 3D
• PolyPos3Dmix_pdf : positive partly symmetric (x<-->y) polynomial in 3D