Add syntax and semantics to mark output ports as required

[bocchino]

Motivation: Sometimes port connections are required for a topology to make sense, e.g., a handler invokes an output port which, if unconnected, causes FSW assertion failure. Other times port connections are not required, e.g., a handler checks whether an output port is connected before invoking it.

Proposal: A port specifier in components can optionally be required, e.g.,

active component C {
  ...
  required output port p: P
  ...
}

In this case it is a topology error to instantiate component C to an instance I and leave port I.p unconnected.