These should help prevent a regression against d9ed74ba

It was intended to set the index of the variable with itself to be NA.
As written, this would set the index of the variable among all variables in the data set to be NA, thus creating longer vectors when the index of that variable exceeded the length of `variables`.
Fixes bug introduced in https://github.com/StephanSeifert/SurrogateMinimalDepth/commit/4039d104f14f5f0a09f1c97135550564163febd6

Now using inherits()

(contributors, those who have made smaller contributions, like patches)

Using recommended defaults for R