- Rename, document the supporting functions for improved clarity
- Drop dim argument from (former) surface_normal

The function registry bits are only relevant for symbolic operators,
and the broadcasting test duplicates what's already being done in
arraycontext.

This ensures that the symbolic stuff remains tested until we remove it.

The issue is that the same expression (dx0/dr0) appears in
both a user provided expression and the discretization
metrics and the code generator gets confused.

Co-authored-by: Andreas Klöckner <inform@tiker.net>

Test that normal DOFArrays for empty boundaries have the same number of groups as the corresponding discretization