Tests for OptionalEnsemble at start of Sequential
Merge remote-tracking branch 'upstream/master' into altered_ensemble

Conflicts:
	opentis/tests/testensemble.py
Bringing Optional and Sliced back to life
Zeroth draft of sliced_sequential_ensemble.ipynb
OptionalEnsemble tests when in middle of SeqEns
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.
SlicedTrajectoryEnsemble.__str__ and its tests.
Most of sliced_sequential_ensembles.ipynb
Completed sliced_sequential_ensembles.ipynb

Also added an extra test, which just showed that I was stupid but the code
was right.
Merge pull request #85 from dwhswenson/altered_ensemble

OptionalEnsemble and SlicedTrajectoryEnsemble