Add `boundary` wrappers

- define `boundary_space(space, name)` function: initially this can just be for the top and bottom of the columns and extruded spaces, and would be equivalent to `level`
- Should fix #639 first
- we could do something fancier where we store the boundary spaces as weak references inside the space, but can do that after.
- define `boundary_field(field, name)` analogously.

_Originally posted by @simonbyrne in https://github.com/CliMA/ClimaCore.jl/issues/325#issuecomment-1083349501_

#639 - currently memory alloc issues, need to perform memory profiling.