Use `sample(index = TRUE)` instead of `isample()`

[GegznaV]

Instead of isample() use sample(index = TRUE):

• [x] implement sample(index = TRUE)
• [x] deprecate isample()
• [x] update NEWS.md

The original discussion is in:

• https://github.com/cbeleites/hyperSpec/issues/177

[GegznaV]

@cbeleites, @bryanhanson, is it a problem that in base R there is no argument index in sample()? I.e., the signature does not have .... Wouldn't sample(index = TRUE) result in some kind of R CMD check failures?

[GegznaV]

OK, here is additional arg drop: So no problems should arise.