Added long-overdue checks for input/output consistency of operands to supported model operators.  Adding this actually invalided most of the test_slicing_on_instances_2 test, since many of the slices it tested actually returned invalid, unevaluatable models (the error message when this occurs during slicing might be worth improving).  I kept test_slicing_on_instances_2, but added a corollary that is essentially the same but doesn't run into this specific problem, so that a few more cases can still be tested.
Added a couple more examples, and reduced the amount of vertical space wasted from the other example plot; fixed more doctests with float output