Add data preparation module for Databricks notebook

[awaismirza92]

Closes #62

[srnnkls]

I'm not giving the same review two times. If you have common functionality to extract, please go ahead and do so. It is not my role as a reviewer to teach you the most basic software engineering best practices (DRY). The idea of separating database-specific logic from orchestration was giving you the ability to look for a common interface, not pseudo-applying lookalike enterprise patterns to simple data preparation scripts for notebook showcases.

See my review on #58 for detailed feedback. The same issues apply here.

Per #42, the target structure is:

getml-demo/integration/{platform}/
├── data/
│   ├── ingestion.py
│   ├── preparation.py
│   └── sql/
│       ├── create_weekly_stores.sql # should probably be called create_stores_per_week or something similar
│       └── calculate_target.sql # should probably called create_weekly_total_sales_per_store_per_week or something similar
└── tests/
    └── test_data_pipeline.py

Specific issues:

• Remove _sql_loader.py (just inline the two lines)
• Remove models.py (define the data structures inline)
• Remove custom exceptions (DataPreparationError adds nothing)
• Consolidate 9 SQL files into 2
• Remove display/analysis functions from preparation.py (this is a data loader, not a reporting tool)
• Remove docstrings and ASCII banner logging