Forbid zero delay.

[HansOlsson]

Closes #3245
Due to the problems with potential systems of equations for zero delay I don't see any other solution.

[henrikt-ma]

Looks good to me. As usual, before providing a review with approval, I'd like to see some sort of language group decision to make such a backwards incompatible change (maybe someone needs to be convinced that there isn't any other solution).

[HansOlsson]

Note that the problematic part has been present since Modelica 1.1.

[eshmoylova]

We check for strictly positive value and throw an error if it is not.

[HansOlsson]

Agreement by acclamation.