WIP: Re-design of the neighbour searching loops and tasks

Changes:

• Collect the max h of the active particles in a cell,
• Redesign the loops over neighbours to make use of this information,
• The sub- and regular tasks change their meaning,
• Added a range of checks for the correctness of the hydro, star and BH loops,
• Push the star ghost to deeper levels,
• Make newly-born stars inherit the smoothing length of their parent gas.

Fixes #696 (closed), #688 (closed)

Todo:

• MPI
• Black hole loops
• Sink loops
• Early abort in subset PAIRS
• Add new recursion parameters to the yaml file