use pydantic instead of dataclass

[eric-downes]

pydantic has a lot stricter typing and should allow us to do more, but we'll need to use pydantic-pint for types because unlike dataclass it does not take type hints as optional.

[eric-downes]

I think we should use pydantic with pint for better unit tracking and conversion, │
but they don't play together very well. My previous attempts to do this failed │
because inside of ode simulations we need unitless quantities for numpy, while │
outside of simulations we want strict pydantic checking of units, so a translation │
layer needs to be built into ethode to extract and pass the values in a typesafe │
way.

Pydantic-pint is supposed to provide this but in practice it is frustrating to use, https://pydantic-pint.readthedocs.io/en/stable/ so we need a better iface and a non-strict mode for quick testing