matlab函数：c2d离散化函数（待完善）

Convert model from continuous to discrete time

sysd =c2d(sys,Ts)
sysd =c2d(sys,Ts,method)
sysd =c2d(sys,Ts,opts)
[sysd,G]= c2d(sys,Ts,method)
[sysd,G]= c2d(sys,Ts,opts)