nonrandom distribution returns a single value when repeated values are expected about simstudy HOT 1 CLOSED

@assignUser I've decided to tackle this issue by allowing customizable distributions. Though I know that we were thinking this would require a whole new infrastructure - maybe even simstudy2 - I took a relatively simple approach here, working in this branch.. Here's an example of the way it is working:

trunc_norm <- function(n, lower, upper, sd) {
  rtnorm(n = n, mean = 0, sd = sd, lower = lower, upper = upper)
}

def <- 
  defData(varname = "x", formula = 5, dist = "poisson") |>
  defData(varname = "y", formula = "trunc_norm", 
    variance = "sd = 100, lower = x - 1, upper = x + 1",
    dist = "custom"
  )

dd <- genData(100000, def)

Of course, it is a little awkward, because we specify the function name in the formula field, and the arguments in the variance field. But that is nothing new for simstudy; indeed, I guess it is part of the charm of simstudy. Adding a new distribution option requires updates a fair bit of documentation/vignettes. I'll start on that, but if you have some major concern/suggestions, feel free to holler.

from simstudy.