Modularize "heart-uci" example for its real-life deployment.

* Add optional parameters to the Python functions and the argparser
  so as to enable running the UCI-Heart-Dataset examples over an
  actual network.
* Make the documentation more verbose about these options, and the
  difference between localhost-based simulations and real-network
  use.
* Rationale: having a minimal example to run is a valuable feature
  when deploying declearn to a new machines~network setting, so as
  to quickly assess (and/or debug) the framework's usability.