SQL scripts as models

[martin-traverse]

SQL model steps take the form of update statements:

UPDATE input_1 SET field_1 = field_1 * 2 WHERE field_2 = "some_filter";

This injects a model into the chain whose output is the input dataset with the update operations applied. The input dataset is not modified - we use the update form because it is clearer than a select when there are a lot of columns, most of which are unchanged.

Implemented by converting into spark sql or operations on a pandas data frame, mostly used for overlays.