Implement models metadata as function attributes

[luisfabib]

Closes #142

Changelog

• Refactor definition of the models.
• Refactor code throughout where necessary.
• Remove model function handle from metadata (implemented in Acceleration of fitting by avoiding model call overhead #101).
• Remove expensive calltree call in model argument parsing (this was the cause of the slowdown seen in DeerLab models incur heavy overhead while fitting #100 and fixed in Acceleration of fitting by avoiding model call overhead #101).