Convert transfer_function_definitions in gen_kw_config from list of strings

[oyvindeide]

`transfer_function_definitions´ is a list of strings that need to be parsed each time, we should instead make this a dataclass/pydantic baseclass so we dont have to parse a string:

Will require migration.

[oyvindeide]

This is also a good opportunity to add logging of which parameter distributions are being used.