Less strict dataflow connection semantics

[mtiller-jh]

Is your feature request related to a problem? Please describe.

When building component models that have input or output signals, the current semantics of MTK make it nearly impossible to create a correct component.

For example, a component that has an input connector that then feeds into the input connector of a subcomponent is an error in MTK currently because the connect doesn't include an output. But it doesn't need to contain an output.

Similarly, an output connector on a component may be connected to an output connector on a subcomponent. This connect has two outputs which is also an error.

Describe the solution you’d like

For building component models, the ideal semantics are those discussed in Section 9 of the Modelica Specification. In particular, Section 9.3 (regarding rules about input and output connectors). This, in turn, relies on the notion of "inside" and "outside" connectors defined in Section 9.1.3.