Only subtasks

Remove the concept of pair and self that directly act at the level in the tree where they are hooked. All pairs are now behaving like the old sub-pair. All selfs like the old sub-self. This allows to simplify the codebase by removing operations that were largely the same.

The diff is best understood in three steps:

• Changes up to 524217c9...d4d334a7 remove the self/pair hydro/star/bh/sink/rt functionality. Only sub-pair and sub-self are left.
• Next changes d4d334a7...b5319e27 directly construct sub-pair and sub-self in maketasks.
• Finally, b5319e27...8b1eeed0 renames all the sub-pair to pair and sub-self to self.