wblr: handling of argument 'x' is too restrictive

Hello David,

I propose to rewrite the following line

https://github.com/Weibull-R/WeibullR/blob/daf3092acf449d148f776fcfa67947e015f18982/R/wblr.R#L92

with 

```r
if (!inherits(x, "data.frame)) {
```

For example, this would enable the use of a `tibble` as argument x or any other extended data.frame.

Best regards
David