Default nonlocal "quadrature rule" = one value
string_to_elem for INVALID_ELEM
Allow any FE to reinit with NULL Elem*

"reinit" for a non-SCALAR FE just effectively means "clear all the
data".

It might have been simpler to have FEMContext selectively reinit only
the SCALAR FE objects, but making them all work is probably more
flexible in the long term.
FEMContext::nonlocal_reinit() can be completed now