Feature Request: logLik method

I was wanting to compare two models using AIC, so I needed a logLik function.  Here it is in case you think it would help:

```r
logLik.xpose_data <- function(object, ...) {
  structure(
    -0.5*as.numeric(rev(object$summary$value[object$summary$label == "ofv"])[1]),
    df=sum(!get_prm(object)$fixed),
    class="logLik"
  )
}
```