Downstream next event tag (DNET), a new signal for more efficient centralized federated execution

[byeonggiljun]

This PR replaces #176 and #337.

Relevant discussion and issue: lf-lang/lingua-franca#1626, #264

A companion PR in lingua-franca: lf-lang/lingua-franca#2400
A companion PR in reactor-ts: lf-lang/reactor-ts#296

In this PR, the new message type, downstream next event tag (DNET), is introduced to reduce unnecessary NET signals. This signal is sent from the RTI to a federate to notify it that NET tags below some threshold are unnecessary, not needed by its downstream federates. Suppose the RTI is computing the DNET tag for a federate A where A has one downstream federate B. The tag is computed by the minimum delay between the federates and B's NET value. For example, if the delay from A to B is NEVER and B's next event is scheduled at (100 ms, 0), the RTI can send DNET (100 ms, 0) to A; if A sends NET (100 ms, 0) to the RTI, the RTI cannot send TAG (100 ms, 0), and if A sends NET (100 ms, 1), the RTI can grant TAG (100 ms, 0) to B.

Additionally, the RTI computes a tag of a tag advance grant (TAG) signal with the earliest incoming message tag (EIMT) to prevent blocking a federate that is eligible to advance its tag. Specifically, if a federate has its next event scheduled at (100 ms, 0) and its EIMT, calculated by the RTI, is (200 ms, 1), the RTI sends TAG (200 ms, 0) so that the federate can execute any potential events with tags earlier than (200 ms, 1) as well as the event at (100 ms, 0).

This PR also contains some refactoring for the data structure used by the RTI to store delays between federates. A two-dimensional matrix stores the delays now.

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[edwardalee]

I need more time to review this, but here is a starting point. This is looking very good so far. I have a few suggestions, some of which are just nits and you should feel free to ignore them. I will follow up with a more complete review as soon as I can.