extract: helper/checker for length-2 atomic vector input

[mdsumner]

A very common error I have made in the past is to input a point for extract and expect a value for a single point:

library(terra)
r <- rast(matrix(1:12, 3, 4))
extract(r, c(2.3, 1.1))

It's a very specific case, input of length==2, numeric input, but I think it's worthy of a message to warn potential mistaken input - especially if either value is outside (1, ncell) or has fractional component. Happy to PR just wanted to record the idea, after a colleague asked me about it, they'd been trying to figure it out for a while.

[rhijmans]

Yes, that is one of these things where R can easily trip you. I have added a check and warning.

[mdsumner]

awesome thank you, just should also check for NA/Inf lest the dreaded "missing value where TRUE/FALSE" needed (I'll also do some more checks)