Marking suitable inputs/outputs as discrete

[Callejn]

The inputs/outputs of Modelica.Blocks.Interfaces.{DiscreteSISO, DiscreteMIMO, DiscreteMIMOs} should be marked as discrete.

[HansOlsson]

No. They should not be marked as discrete - and this pull request shall not be merged.

The discrete keyword in Modelica indicate that the variable is assigned in a when-clause; but a variable can be discrete in a broader sense without this keyword.

The inputs are clearly not given by a when-statement (since their values are given by a connect-statement).

In some cases it is right for the outputs - but it depends, e.g. it is wrong for: Modelica.Blocks.Discrete.ZeroOrderHold - and likely also for user models.

See also #2123

[HansOlsson]

Reject addition of 'discrete'. The issue is that discrete in Modelica as a keyword mean assigned in a when-clause, and is not even needed in that case. For the inputs it is never correct (they are connected to). For the outputs it is sometimes incorrect (and often correct); and thus this would break some existing models (including one in MSL). The reason for 'discrete' just meaning 'assigned in when-clause' is that it is straightforward to describe and test - whereas a more general 'discrete' would require specifying a decomposition of models etc. From a user-perspective "discrete" is not that intuitive to use, if we don't know when it changes, that is improved with the synchronous addition in Modelica 3.3.

[HansOlsson]

However, the fact that we have gotten two reports about this issue indicate that we might add the explanation from #2123 to the specific components.

[dietmarw]

This PR is superseded by #2271