Streamline Error Propagation for working by slice in banzai-nres

[markBowman]

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