Remove the concept of the PROJECT_ROOT

[csala]

No python code should have to rely on its location, so having to declare a PROJECT_ROOT variable in the main package is a sign of weak points in the project.

In any case, any path variables should be computed in the scripts that use the atm package, not within it.

[micahjsmith]

This goes hand-in-hand with an approach to rethink how configuration is supplied for the project.

Here is an example from Apache Airflow:

• The path to the configuration file and work directory is computed: https://github.com/apache/incubator-airflow/blob/master/airflow/configuration.py#L425
• The configuration file is read using ConfigParser https://github.com/apache/incubator-airflow/blob/master/airflow/configuration.py#L510
• Any application code that needs configuration variables (like data directory etc.) does from atm import configuration as conf and then conf.get('DATA_DIR'): https://github.com/apache/incubator-airflow/blob/master/airflow/settings.py#L35