An object is a mooring if it is an unnamed list and if each
element in that list is a mooring element created by
e.g. anchor(), wire(), or a related function.
For example, the
output of mooring() is a mooring, but the output of anchor() is not.
This function is mainly designed for use within the package that that, e.g.
knockdown() will produce an error if its first argument is not a mooring.
is.mooring(m = NULL)