Add type hints to python code

[damccorm]

As a developer of the beam source code, I would like the code to use pep484 type hints so that I can clearly see what types are required, get completion in my IDE, and enforce code correctness via a static analyzer like mypy.

This may be considered a precursor to BEAM-7060

Work has been started here:  #9056

 

 

Imported from Jira BEAM-7746. Original Jira may contain additional context.
Reported by: chadrik.