Users struggle with the difference between a Catalog and a computed NestedFrame

Recent user feedback has shown confusion between the catalog object and the nested frame once a catalog has been computed. This includes trying to do catalog operations such as crossmatch on NestedFrames and running into method not found errors or missing attributes like meta which don't help tracking down the problem.

We could either try to make error messages clearer on NestedFrame to hint that the user is not using a Catalog, have clearer explanations in the docs on this issue, or go as far as creating a custom `ComputedCatalog` class that is returned from compute that the user can use as a NestedFrame and also do lazy operations on to go back to a `Catalog`.