Support for factor covariates

[jeremyrcoyle]

Currently nonexistent. One implementation exists here: https://github.com/jeremyrcoyle/sl3/blob/tmle-demo-fixes/R/sl3_Task.R#L275, however this is done on-the-fly every time task$X is called. This results in repeated computation for the indicators when fitting lots of models to the same task. We can do better.