Lazy Loading

Migrate study pipeline to lazy loading, simplify dataframe validation using Pandera. Raw file reads are now cached_property accessors and can be unloaded after extraction. _load_data() becomes obsolete. parquet I/O consolidated in data_store.py.

What's Changed

• 143 lazy loading by @janvv in #149
  Full Changelog: v0.2.1...v0.2.2