Use __init__.py and absolute (path) import and restructure the project.

[K-Wu]

Parent issue: #1

An empty init.py marks its directory as a module.

I propose the code to be restructured into

Readme.md

......
pylog/optimizer.py
pylog/pylog.py
tests/......
......

In pylog/pylog.py, optimizer.py can be now imported by absolute import statement

import pylog.optimizer

This enhancement much clarifies the import statements.

https://github.com/UCLA-VAST/soda is an example.