Streamline Error Propagation for working by slice in banzai-nres #118
Description
As a pipeline developer, I want to be able to use error propagation when taking a slice of data, e.g. a single order for NRES, so that the error propagation is robust (actually applied without duplicated code)
- Add slice methods to banzai
- Integrate new error propagation into the flat field divided extraction for banzai-nres
- Test that the errors produced are reasonable
- merge to dev