In order to provide full Lean Indicator functionality to python custom indicators, they need to inherit from a C# class. `PythonIndicator` will serve for this purpose.
Algorithms can use the former version (no inheritance).
In order to add support custom python indicators for `QCAlgorithm.PlotIndicator`, we created a `PythonIndicator` class that wraps the custom python indicator. In `QCAlgorithm`, the reference of the wrapper is saved into a dictionary keyed by the python indicator handle.