Re-examine Field Bundles

There currently exists the concept of a "field bundle." It is an array of fields with special meanings. In particular, which of the fields is the pressure field is specified by a global scope integer. These fields are then iterated over.

It is used high up in the algorithm layer so ultimate performance is less of a concern.

It looks like we could do better here, maybe using a field collection. These did not exist when field bundle was created.

The concept of "field bundle" can also be found in the core science component. Maybe it shouldn't be, maybe it should use a field collection. This may turn out to be a linked issue.

Locate the field bundle definition and its uses in the algorithm layer and core science component. Read how field collections represent the same data, then determine whether both uses should be migrated; done means the replacement or decision is documented and the affected uses are consistent.

Written by the indexing model from the issue text.