Seismometer config properties should pass through to Config

[gbowlin]

We have a new pattern of referencing Config as an immutable object.

    @property
    def target_cols(self) -> list:
        return self.config.targets

We should do this for other config properties like cohort_cols to that they remain immuatable.

        self.cohort_cols: list[str] = []

Originally posted by @gbowlin in https://github.com/epic-open-source/seismometer/pull/30#discussion_r1653326296