add unique, unique.byrow, unique.bycol to check_matrix() and friends

[maxheld83]

check_vector() helpfully provides a unique argument.

It would be really nice to have this for two-dimensional objects as well, where it's often important to be unique overall, or by row or by column.

Perhaps something like:

# helper: check unique by column
check_unique_in_column <- function(x) {
  duplicates <- apply(X = x, MARGIN = 2, FUN = function(x) {
    duplicated(x = x, incomparables = NA)
  })
  if (any(duplicates)) {
    return("must only have unique entries by column")
  } else {
    return(TRUE)
  }
}
assert_unique_in_column <- makeAssertionFunction(check_unique_in_column)

I am aware that much of the stuff that checkmate does under the hood is written in C, which unfortunately I cannot do. If any of these additions could also be helpful in R, I'd be happy to open respective PRs, though I might need some guidance to make sure any potential additions sit well with the overall design and infrastructure.