Decompose rational function with repeated linear factors (#565)

Decompose rational function with repeated linear factors (#565)

Express P(x)(x−a)n\frac{P(x)}{(x-a)^n} as A1x−a+A2(x−a)2+⋯\frac{A_1}{x-a} + \frac{A_2}{(x-a)^2} + \cdots.