Consolidate field_subset.offset and .shift

There are two types of domains of a field that we currently support, affine subspaces, and multiplicative cosets.

Currently `.offset` is only used in the additive subspace case, and `.shift` is only used in the multiplicative subgroup case.  It'd be best to consolidate into only using such method for code consolidation/clarity.

I suggest using `.shift` or `affine_shift`