Fix background correction path problems

[talonchandler]

In my initial prototypes my background corrections strings weren't being passed correctly. After discussing with @ziw-liu and @edyoshikun I think I need to use str type and validate. I expect this to be a fairly shallow bug.

[edyoshikun]

something like this

from pydantic import BaseModel, validator, DirectoryPath

class _BirefringenceApplyInverseSettings(BaseModel):
background_path: str = ''
    @validator('background_path')
    def check_dir_path(cls, v):
        return DirectoryPath().validate(v)

[talonchandler]

Fixed by #353