Replace present_to_schema with macro in examples and docs

[jmatsushita]

Fixes #34

Looking at this test it seems that the way to call present_to_schema is to use the macro @present_to_schema

• Fixed 2 notebook examples:
  • examples/ml_workflow_demo/ml_demo.ipynb
  • examples/company_demo/concise_demo_ACT.ipynb
• Fixed README.md

It might be possible to write doc tests and use NBInclude.js to tests the notebooks as well (although that might mean either disabling the integration cell which use Postgres, or setting up a more involved testing suite).

Happy to give it a try here if that's of interest or do it in a subsequent PR.

Cheers,

Jun