trim_ps.RdFunction for symmetric trimming predicted probabilites
trim_ps(ps, trim)a matrix of predicted probbilities, each row should sum to 1
a scalar that providing an upper and lower timming threshold as a percentile. For example, trim = 0.01 will drop the observations with propensity scores below to the 1st percentile and above the 99th percentile.
Returns a vector of Boolean variables identifying which rows in the data to keep