Type Checking Bug on Sparse Vertical Index Fields

[Stagno]

Currently, the type checking on index fields is too strict. Consider:

@stencil
def a_stencil(
    a: Field[Cell, K],
    b: Field[Edge, K],        
    test_field: IndexField[Cell > Edge, K]
):
  with domain.upward as k:
    a = sum_over(Cell > Edge, b[test_field])      

this is rejected by the dimensions checker because the types of b and test_field are different. However, this is clearly a sensible example. The type checking should not enforce that b and test_fieldhave the same type, but that both types are valid in the current iteration space.

NOTE: This issue was updated because it was initially thought that sparse index fields are not supported at all.