This implements the fourth heuristic of the Maple Paper.
The main aim of this heuristic is to solve the partial differential equation
chi.diff(x) + h*chi.diff(y) - hy*chi = 0 for chi and set values
for eta and xi such that eta = xi*h + chi

It assumes chi to be a bivariate polynomial in x and y
Added delimiter
Changed .is_Rational to has(x) or has(y)
Reverted to old algo for calculating degree
Merge pull request #2214 from Manoj-Kumar-S/heuristic4

Heuristic 4