matlab函数：c2d离散化函数（待完善）

Convert model from continuous to discrete time

sysd =c2dsys,Ts)
sysd =c2dsys,Ts,method)
sysd =c2dsys,Ts,opts)
[sysd,G]= c2dsys,Ts,method)
[sysd,G]= c2dsys,Ts,opts)