design.matrix: Generate design matrix for two-way factorial analysis
In TANOVA: Time Course Analysis of Variance for Microarray

This is an internal function to be called by ls.estimate to generate design matrix. Users should call ls.estimate whenever is possible.

1	design.matrix(f1,f2)

`f1`	a vector of length equal to the number of arrays. Each entry indicates the level of the first factor for corresponding array. The values of f1 should be 1,2,3,...
`f2`	a vector of length equal to the number of arrays. Each entry indicates the level of the second factor for the corresponding array. The values of f2 should be 1,2,3,... If the experimental has only one factor, let f2=0.

The output is a list object.