Internals up for deprecation

[gmbecker]

tt_level and tt_level<- - Only used in summary functions, 'level' appears to be ignored by printing machinery

clayout_splits and clayout_splits<- - Not used anywhere and I'm pretty sure the mental model is wrong anyway as it assumed a single nested hierarchical splitting structure, which is almost always the case but NOT guaranteed/enforced.

split_rows_by_multivar - apparently this is not and has never been exported.

[gmbecker]

Closing as stale