Return sum of prolated duration of all leaves in chain.
Todo
Write tietools.get_prolated_tie_chain_duration() tests.
Changed in version 2.0: renamed tietools.get_duration_prolated() to tietools.get_prolated_tie_chain_duration().
Changed in version 2.0: renamed tietools.get_tie_chain_prolated_duration() to tietools.get_prolated_tie_chain_duration().