Add max_unique parameter to all metadata functions

[lhdjung]

Currently, only mode_is_trivial() and mode_count_range() have a max_unique parameter. I think all metadata functions should have one. The remaining ones are:

• mode_count()
• mode_frequency()
• mode_frequency_range()

Why do they need the parameter? Consider x <- c(7, 7, 8, 8, NA). Assuming that the NA is known to be either 7 or 8, i.e., max_unique = "known", the functions currently deal with x the wrong way because they are unsure whether NA represents one of the known values or not. If they knew that it does, they would work differently:

• mode_count() would be able to conclude that NA breaks the tie so that the mode count is 1.
• mode_frequency() would see its target statistic rise to 3.
• mode_frequency_range() would know that the maximal frequency is the actual one and return c(3, 3); just like mode_count_range(x, max_unique = "known") already returns c(1, 1).

However, this is just a corner case: a single value is missing, and all known values are equally frequent. max_unique doesn't matter to these functions otherwise, except for mode_frequency_range() if all known values are equal.

I don't think that max_unique matters at all to mode_first(), mode_all(), and mode_single() – the functions that attempt to find actual modes. The metadata functions don't, which is why they are able to gain any information from vectors like x at all. In other words, mode_all() and friends would fail x anyways, so max_unique wouldn't help them.

I have not thought about mode_possible_min() and mode_possible_max() in this context. Would they benefit from max_unique?