internal function to simulate data for examples
.sim.data(n = 500, m = 10, beta = 0.2)number of independent observations
group size - number of times each independent observation is repeated
Y1 ~ N( beta * X, 1)
data.table with Chr (always 1, possibly needed for bootstrap), x (explanatory variable), y1 (response variable related to x), y0 (response variable unrelated to x) name (unique name for each independent observation)