Make getSuff work with tibbles

[BjarkeHautop]

Accessing using [[i]] is safer and works with tibbles too.

Issue before:

mwe <- tibble::tibble(
  x = factor(c(rep("a", 5), rep("b", 5))),
  y = factor(c(rep(c("a", "b"), 5)))
)

micd::mixCItest(1, 2, NULL, suffStat = as.data.frame(mwe))
#> [1] 0.5256929

micd::mixCItest(1, 2, NULL, suffStat = mwe)
#> Error in `getSuff()`:
#> ! 'list' object cannot be coerced to type 'double'

^{Created on 2026-06-17 with reprex v2.1.1}