add function.AssertEquals for shapes

[gertjanvanzwieten]

Currently operations with more than a single argument must have identical shapes, including when shapes contain functions. The problem with that is that functions have no canonical form (even though an attempt is made using simplify) so that check is too strict. A solution would be to introduce an AssertEquals operation that evaluates both, asserts equality, and returns the result. Its simplify operation can do what is done currently: simplify the arguments and return one if it matches the other.