should have a function to attach elements to a wire

[dankelley]

If an instrument is attached to a wire via a clamp, its height value ought not to be added to the total length of the mooring. For deep-ocean moorings with say 10 instruments of height 0.5m each, this is not an issue that matters much. However, it will matter for short moorings (like a 2-metre mooring of issue #47).

Some instruments have words like 'clamp on' in their names, but still have nonzero height values within their description. I suppose it would be worth changing that, but there is also the possibility of a new function to indicate that something is clamped on.

This could be a function like the following

clampon <- function(i) {
   i$height <- 0
   i
}

[dankelley]

I added clamped() to the "main" branch, commit 7b50baf; see the examples for ?clamped (or https://dankelley.github.io/mooring/reference/clamped.html) to see it in action.