Heuristics, for assumptions on xi and eta

Implemented the following hints
1. Function_Sum:
   It assumes `eta = 0, xi = f(x) + g(y)` ; `xi = 0, eta = f(x) + g(y)`
2. Abaco2_Similar:
   It assumes `eta = f(x) ,xi = g(x)` ; `eta = f(y), xi = g(y)`
3. Abaco2_Unique_Unknown:
   It assumes `eta = f(x) ,xi = g(y)` ; `eta = f(y), xi = g(x)` , when there are unknown functions or functions with non-integer powers.
4. Abaco2_Unique_General:
5. Linear