Allows using the `run` function from tests and examples.

Implement runtime-provided periodic timer input for source operators

Ensures that the event is only sent after all input straems are exhausted. This avoids a potential race when there are multiples inputs.

The dora timers produce values forever. We only want to stop operators when all their input streams are closed.

Python Node API

Since the merge of https://github.com/PyO3/maturin/pull/605 ,
we can add features flag when maturin build the python project, removing
the need to always enabling `extension-module` flag when building.

The `extension-module` flag creates crash when building and testing outside
of maturin.

Previous fix required to disable default feature flag when building and testing which is not ideal.

Adding `next` and `send_output` requires an async threadpool as the
communication Layer defined by the Middleware Layer returns an async Future
stream.

I solve this issue by adding a tokio runtime on a separater thread that is connected with two channels.
One for sending data and one for receiving data.

Those channel are then exposed synchronously to Python. This should not be cause for
concern as channel are really fast.

Looking at Zenoh Python client, they are heavily using `pyo3-asyncio` implementation
of futures to pass Rust futures into Python.

This can be a solution as well, but, from previous experiment, I'm concerned about performance on such
solution. I have experienced that putting futures from Rust into the `asyncio` queue to be slow.

I'm concerned also by mixing `async` and `sync` code in Python, as it might be blocking. This might requires 2 threadpool in Python.
This might seem as heavy overhead for some operations.

The `Send` Trait is required by pyo3 for the python binding

Add support for single operator nodes in YAML dataflow specification

Moving folders in correspondance with #41

Simply moving crate to mirror our discussion without touching at names

Use `cargo run` instead of the path of the binary in the
dataflow as recommended way of using `run:` as it is:

   -OS agnostic
   -Compile node before being run which avoids the risk of missing development changes
   -More readable

Injecting the env variable diclared in the yaml into the node runtime.

To do that we need to re-serialize the parsed env variables into
string.

Expose name of tracing and fix install batch issue