python API docs

[Phlya]

We should create them...

From highest priority to lowest:

• io/header: https://github.com/open2c/pairtools/blob/master/pairtools/lib/headerops.py
• scaling: https://github.com/open2c/pairtools/blob/master/pairtools/lib/scaling.py
• stats: https://github.com/open2c/pairtools/blob/master/pairtools/lib/stats.py

[agalitsyna]

Currently, not all lib modules should be considered user-exposable APIs.
Only io/header, scaling, and stats can be helpful when accessed from Python. Maybe we can document them; I agree!
If you think other lib modules can be useful, feel free to add them.