Codegen

[TK-21st]

Major refactor of the package.

Major design changes include:

1. Migrating to Numba for codegeneration, which removes dependency on pycodegen. No codegen is required beside formatting methods before calling numba's jit decorator
2. Backends are designed as Mixins and directly replaces model methods for maximum performance and quality of life for users.
3. Every model instance has associated num and is therefore aware of the number of components being instantiated, and model attributes are wrapped in a structured array called _data. This significantly simplifies codegeneration.