PAC model: make detection of non-optimizing part more robust

Introduce a new method for decomposing a product of factors, so that we can
identify expressions of the form (1-optim_share)*A*B.

Also enforce that the optim_share parameter be in a factor of the form
1-optim_share (previously it would accept any expression containing the
parameter).

Note that this fix does not yet allow to actually write non-optimizing parts of
the form (1-optim_share)*A*B, since at a later point the preprocessor imposes
that this part be a linear combination of variables (but in the future we could
think of expanding the A*B product into a linear combination if, for example, A
is a paramater or a constant and B is a linear combination).

Closes: #50