Support complicated dependency relationship #16

## Feature Request

Description of Problem:

In current datahub_core, we only have linear regression model which has input dependencies from other fields. Right now, we only support the dependency relationship in a simplified way in this PR: #15. It's possible that the output of the linear regression model will also be used to calculate some derived fields. We need to find a way to support complicated scenarios.

Potential Solutions:

We can probably introduce the concept of derived field/column in the generation workflow.