Config files to control order of operations

A problem with the current implementation of `fileio.open_dataset` is that the order of operations (i.e. general selection then spatial selection/aggregation then temporal aggregation, etc) is fixed.

One way around this might be an approach @dougiesquire is using, where the order of operations can be set by listing relevant function names in a YAML config file. e.g. https://github.com/dougiesquire/Squire_2022_CAFE-f6/blob/main/data/config/CAFEf6.yml

Start by reading the current fileio.open_dataset implementation and the linked CAFEf6 YAML configuration example. Identify the existing fixed order of general, spatial, and temporal operations, then define how a configuration would express that order. Done means operations can be reordered through a YAML config without losing the existing selection and aggregation behavior.

Written by the indexing model from the issue text.