Add `summary` function

[larryshamalama]

Here is a sketch of what and how the existing code would have to be modified

causens <- function(...) {
  ...
  result <- list(estimated_ate = estimated_ate)
  class(result) <- "causens"

  return(result)
}

The following works via S3 dispatch

summary.causens <- function(causens_object, ...) {
  cat("Summary of Causal Effect Analysis\n")
  cat("=================================\n")
  cat(sprintf("Estimated Average Treatment Effect (ATE): %f\n", causens_object$estimated_ate))

  invisible(object)
}

[larryshamalama]

Probably good to also do this with plot