Simple Error in measurement_error.R

Hi --

I found what I think is a pretty simple error in measurement_error.R.  In the following block of code, $lwr is incorrectly written into upr: 

if ('upr' %in% names(extra_opts)) {
    upr <- extra_opts$lwr
  } else {
    upr <- 0.75
  }

Suggest the following quick fix: 
  upr <- extra_opts$**lwr**
        ----------->
  upr <- extra_opts$**upr**

