ODE: Added non homogeneoous linear euler eq fixes #7172

The issue https://github.com/sympy/sympy/issues/7172, which is about implementing non homogeneous euler differential equations like ```{a_n}*x**n*diff(f(x), x, n) + ... + {a_1}*x*diff(f(x), x) + {a_0}*f(x) = g(x)```. 
This PR contain two methods to implement it,
One is undetermined coefficients which is used after converting this non homogeneous equation into homogeneous one by putting ```x = exp(t)```.
Second is variation of parameters which can be used directly after dividing the coefficients of nth term. This involves first solving the homogeneous part by homogeneous_euler_eq and for non-homogeneous part using variation of parameters solves the equation.