[ENH] Default `_iloc` and `_iat` to include handling of composite distribution objects

[fkiraly]

In a number of PR, it became necessary to write custom _iloc and _at, in a couple different cases of composite distributions: #555, #557.

The boilerplate to subset distributions always seems the same (simply calling their respective iloc), so it feels like this should be the default for handling distribution objects.

The current default _iloc handles array-like objects, so we could add another branch to the logic handling distribution objects (after checking type) the same way.