Add tip in error message because of non-continuous traits

[Rekyt]

It's the second person that contacts me because of the non-continuous trait error message:

fundiversity/R/fd_fric.R

Lines 65 to 68 in 0a58ab6

	if (!is.numeric(traits)) {
	stop("Non-continuous trait data found in input traits. ",
	"Please provide only continuous trait data", call. = FALSE)
	}

Maybe we should point people to multivariate analyses to get back continuous traits. Like adding a line "If you want to use non-numeric traits with fundiversity, you have to transform them to obtain numerical traits beforehand (e.g., through a PCoA or similar techniques)"

It would probably be too specific, but at least would point user to ways to overcome the issue themselves.

[Bisaloo]

For this kind of situation, instead of creating super long error messages, especially with potentially subjective information, I like to create a FAQ vignette.