Function that extracts the last name in a string representing an object as in e.g. obj$x$y.
It returns a list containing both the piece before the last occurrence of $ and the name coming after it.
The object may or may not exist.
extract_root_and_last_member(full_name)character scalar from which we should keep the piece that comes after the last $ sign.
A list with two members: root containing the path to the last name of the object expression
and name, containing the last name in the object expression.
The root is empty when there is no $ in the input string.
Ex: extract_root_and_last_member("obj$x$y") returns list(root="obj$x", name="y").