Add blocks: Multiply, Divide, Add, Subtract

[eltos]

To learn how to implement blocks in gr4 I want to implement and contribute a simple math block as part of the Block Tutorial at European GNU Radio Days 2024. This PR is a work-in-progress state of my efforts.

• Implement the following blocks from EPIC: List of GR 3.10 blocks & Others to be ported to GR 4.0 #161 which take N inputs and combine them into a single output using one of four operations
  • Add
  • Divide
  • Multiply
  • Subtract
• Test and debug locally
• Add unit test (QA)

Usage example:

gr::Graph graph{};
// source_1 counting 1,2,3,4,...,10
auto& source_1 = graph.emplaceBlock<gr::testing::CountingSource<float>>({{"n_samples_max", 10U}});
// source_2 counting 6,7,8,9,...,15
auto& source_2 = graph.emplaceBlock<gr::testing::CountingSource<float>>({{"n_samples_max", 10U}});
source_2.default_value = 5.0f;
// Add block with two inputs
auto& adder = graph.emplaceBlock<gr::math::Add<float>>({{"n_inputs", 2U}});
// Null sink
auto& sink = graph.emplaceBlock<gr::testing::CountingSink<float>>();

// Connections (note how the two inputs of the adder block are addressed)
auto connected = true;
connected &= graph.connect<"out">(source_1).to<"in", 0>(adder) == gr::ConnectionResult::SUCCESS;
connected &= graph.connect<"out">(source_2).to<"in", 1>(adder) == gr::ConnectionResult::SUCCESS;
connected &= graph.connect<"out">(adder).to<"in">(sink) == gr::ConnectionResult::SUCCESS;
if (!connected) { fmt::print("Error: Failed to connect flowgraph!\n"); return; }

// Execute flow graph
gr::scheduler::Simple<> scheduler{std::move(graph)};
scheduler.runAndWait();

Feedback:

I found it difficult to implement the block with a dynamic number of ports since I didn't find documentation on this, nor did I find a description on how to connect the ports in the graph (tried "in0", "in#0" etc. first).
I finally got it, but since this is a very general concept, it would be beneficial to provide the mechanics of "n_inputs" settings etc. in the base block and enable something like PortInMulti<T, Limits<1, 32>> in; for ease of use.
Also, having more example flowgraphs like the one above would be helpful for newcomers.