Fix recursive algorithm to work on every summand
So far discretization algorithm work more or less like this:
1. Expand intput expression to obtain Add-s of Mul-s
2. For every summand do:
* split into lhs, operators, rhs
* calculate derivative
* pass it as input to 1.
3. gather stuff together
Idea to fix it as following:
- First expand
- loop over summands to keep them separated.
- For every summand launch recursive algorithm that discretize it (point 2 above)
- short hoppings
- gather results from summands (after the loop from 2).
Note1: I can do it Note2: shortening hoppings should be a separate function -> another issue