modelMatrix takes in a data.frame and encodes it as a sparse model.Matrix object.
Factors use treatment ("one-hot") encoding, creating indicator variables from categorical
and binary variables.
Note that names of categorical variables will change slightly 
(n-1 unique values appended to name)
1  | modelMatrix(x, interactions = FALSE, intercept = FALSE)
 | 
x | 
 A data.frame with finite, non-missing elements.  | 
interactions | 
 Logical; if   | 
intercept | 
 Logcal; if   | 
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