Reactive Engine

Checklist

- [x] a working version of reactive engine
- [x] binding outputs to client side with an example
- [ ] remove unneeded dependencies like pandas or intra-package
- [ ] add suspend/resume feature for observers + tests
- [ ] add isolated value extraction and isolated environment feature + tests
- [ ] write tests for UndefinedKey and pending observers/expressions
- [ ] full test coverage on reactive.py
- [ ] add memoization/caching of expressions
- [ ] add md5 hasher for Python base types + numpy and panads
- [ ] track potential inplace reactive value changes e.g. via list.remove(), add safe/unsafe mods