Add multi-level subdivision support

[arkid15r]

Many entities have multi-level subdivision structure (see #1710 request for adding Barcelona holidays, where province ES-B has autonomous community ES-CT as parent -- https://www.iso.org/obp/ui/#iso:code:3166:ES).

As long as the structure is defined by ISO3166-2 it'd be significant improvement to have multi-level subdivision support. We've added subdivision alias support recently so it won't be an issue to have Barcelona -> B mapping.

A couple of aspects to think through:

• parent/child relation implementation
• parent subdivision holidays population approach