Live-cells-py

* Fix issue with scheduled watch function:

Incorrect values were being read for referenced argument cells.

* Fix issue with unit tests being installed as `tests` package.

* Fix bugs in examples in README

* Fix issue with argument cell tracking on green threads.

* Add installation instructions to README.

Initial Release.

The following features are ported from Live Cells Dart:

* Mutable Cells
* Computed Cells
* Batch Updates
* Watch Functions
* Peek Cells