Add check that sparse fields are accessed offset in nested reductions and non-offset anywhere else

[Stagno]

Considering the following field (or temporary field):

sparse_field: Field[Cell > Edge]

The following ways to access it are non-offset:

with sparse[Cell > Edge]:
   sparse_field = ... sparse_field...

sum_over(Cell > Edge, sparse_field)

while to access it offset, you would need nested reductions:

sum_over(Edge > Cell, sum_over(Cell > Edge, sparse_field))

Add a check (and tests) to enforce that the access is correctly tagged as offset/non-offset (i.e. offset iff inside nested reduction).