Make `FlavorTransformation` produce a transformation matrix

[Andrey Sheshukov]

Currently FlavorTransformation can only produce prob_ee, prob_ex, prob_xe, prob_xx (and their bar counterparts).
I think it would be useful to have a method to output a transformation matrix, for all the four flavors, like this:

M = [
[prob_ee, prob_ex,          0,          0],
[prob_xe, prob_xx,          0,          0],
[      0,       0, prob_eebar, prob_xebar],
[      0,       0, prob_eebar, prob_xebar]]

and its permutation depending on the order of our Flavor enum.