reflectometry.model1d.tests.opticalmatrix: Pure numeric python implementation of the optical matrix formalism
for computing reflectivity as a function of Q and lambda.
reflectometry.model1d.tests.parratt: Pure numeric python implementation of Parratt's recursion for
computing reflectivity as a function of Q and lambda.
reflectometry.model1d.tests.test_reflAtExtreme: Cross check the extreme cases:
1) sigma=0 matches sigma=1e-10
2) reflectivity at Q==0 is 1
3) d=0 is ok, and about equal t0 d=1.0e-10
4) no layers works (only incident and substrate)
5) polarized calculations match non-polarized when rho_m is 0.