Combine `NCCatalog`s

For calibration, one way to organize data is to have each `NCCatalog` corresponds to a particular dataset or artifact. There are cases where you want to use data from multiple `NCCatalog`s, so having an uniform way of accessing the data through one object as opposed to multiple objects would be nice.

The only problem is that the preprocessing is attached to a struct that wrap `NCCatalog`, so this might not be as effective.

Start by locating the NCCatalog definition and the struct that wraps it with preprocessing. Trace how catalog data is currently accessed and how preprocessing is attached, then determine whether one combined object can preserve that behavior across multiple catalogs. Done means the design is settled and a uniform access path is defined without losing preprocessing.

Written by the indexing model from the issue text.