[FEATURE] Define GTFS Data Models with basic DB-like functionality

[e-lo]

User Story

As a developer, I'd like to rely on GTFS being in a specific format with specific types.
As a developer, I'd like abstract basic low-level functionality like foreign key validation to something happening in background.
As a user, I'd like to get clear error messages when my data is not valid so I understand what is wrong and how to fix it.

Priority

High

Level of Effort

Med-High

Resolution

• Define basic GTFS tables as a Pandera schema
• Define additional needs for Wrangler as subclasses of GTFS Pandera Schemas and write converters between them
• Write custom Mixin class models/_base/db/DBModelMixin which abstracts foreign key validation and other generalizable tasks.

Tests

Tests are in /tests/test_models

[e-lo]

Fixed in #327