Allocating fields on sets

Is your feature request related to a problem? Please describe.
Add the capability to allocate data fields on mesh sets (e.g. element, face, vertex sets). This is needed for efficient implementation of boundary conditions that are not prescribed directly on DOFs.

Describe the solution you'd like
Proposed implementation: in ObjectManagerBase -> "sets" subgroup replace stored view wrappers of index sets with another level of subgroups that will contain the index set view wrapper along with any data fields and will be properly resized. Any loop over such a set (e.g. in BoundaryConditionBase) should pass to the user callback lambda both the mesh index and the index-within-set of the cell/face/vertex, so that user code can properly access both the globally-allocated and the set-allocated fields.

Describe alternatives you've considered

• Always allocating the fields on the full mesh (wastes a lot of memory)
• Rewriting boundary condition loops so as to avoid storing data (leads to more messy and potentially slower code)

Additional context

Start by locating ObjectManagerBase and its "sets" subgroup, then trace BoundaryConditionBase loops and their callbacks. Review existing index-set view-wrapper resizing and field allocation behavior; done means set-specific fields resize correctly and callbacks expose both the mesh index and the index within the set, with corresponding tests or examples passing.