/kiinteisto-*/features/v2 Restructuring sub features on complex feature products

[teezip]

BETA 1 (2021-03) release as described on #3 contains simple feature products, but also some complex feature products, like "RekisteriyksikonLaajatSijaintitiedot" and "PalstanLaajatSijaintitiedot".

Samples
https://beta-paikkatieto.maanmittauslaitos.fi/kiinteisto-avoin/features/v1/collections/RekisteriyksikonAvoimetOminaisuustiedot/items/12910118?crs=http://www.opengis.net/def/crs/EPSG/0/3067

https://beta-paikkatieto.maanmittauslaitos.fi/kiinteisto-avoin/features/v1/collections/RekisteriyksikonLaajatSijaintitiedot/items/157381921?crs=http://www.opengis.net/def/crs/EPSG/0/3067

Currently sub features below properties of the main feature are not encoded as valid GeoJSON Feature elements.

So suggested that sub features would be encoded as GeoJSON Feature elements (with type="Feature", id, properties and geometry).

This would help some user apps - same code to read for example boundary (KiinteistorajanSijaintitiedot) from a separate product for boundary or from a complex product in which a boundary is a sub feature.