ROOT_PATH set automatically when importing visit_processor

[TheSkyentist]

ROOT_PATH, which controls the directory of (most) execution is set when the visit_processor module is imported:

https://github.com/gbrammer/grizli/blob/1ed492f4395add1cbcaa727f4b7f3b242e80979b/grizli/aws/visit_processor.py#L18-L20

However, it would be helpful to allow the user to (optionally) pass this as a parameter when calling functions from visit_processor.

This would apply to the following functions in visit_processor:

1. get_assoc_yaml_from_s3
2. check_jwst_assoc_guiding
3. process_visit
4. get_wcs_guess
5. run_one

[TheSkyentist]

Closed due to reasons outlined in the comments to #222