Implementations for as_offset

[nfarabullini]

Implementation of

 @field_operator(backend=fieldview_backend)
  def offset_index_field_fo(
      a: Field[[IDim, KDim], float64],
      offset_field: Field[[IDim, KDim], int64],
  ) -> Field[[IDim, KDim], float64]:
      return a(as_offset(Ioff, offset_field))

where as_offset shifts a field of values wrt to a field of offset values

[DropD]

Just a small clarity improvement (probably copy pasta)

[havogt]

Should this be in a ffront/common.py or something?

[nfarabullini]

if I move it to commonI get this error:

    dim: func_common.Dimension
E   AttributeError: partially initialized module 'gt4py.next.common' has no attribute 'Dimension' (most likely due to a circular import)