Fixes for OptEns.call at end of Sequential

This required adjusting SequentialEnsemble a bit. This and the remaining
tests of OptionalEnsemble at the end of a SequentialEnsemble may be
difficult, so I'm committing them one at a time so I can revert if I screw
things up.
Tests: OptEns.can_append at end of SequentialEns
Tests: OptEns.can_prepend at end of SequentialEns
OptionalEnsemble tests complete and passing.