This function creates a design matrix for the g(0) or scale covariates using
the input model formula. It returns a list which contains 2 elements: 1)
dim: the dimension (number of columns) of the design matrix, and 2) cov: the
constructed design matrix. This function is relatively simple because it
uses the built-in function model.matrix which does the
majority of the work. This function handles 2 exceptions "~.", the null
model with 0 columns and "~1" the intercept only model - a column of 1s. If
a model other than the 2 exceptions is provided, it calls
model.matrix to construct the columns. If any of the columns of
the design matrix are all 0's the column is removed. This occurs when there
is no data for a particular factor.