Draft: Subtask speedup - Still requires work

For now, compile with CFLAGS=-DONLY_SUBTASKS added to the configuration line.

This introduces significantly faster neighbour finding in particle distributions with strong density gradients. For instance planetary applications or problematic feedback-disturbed galaxies.

Changes:

• Add the brute-force density checks to the planetary scheme (not just sphenix)
• Rewrite the recursion logic in the hydro and stars sub-task:
  • The interaction functions have extra parameters to optionally consider particles only between 0.5 * width and width
  • The subtask recursion now continues to lower level if we reach a level where h is too large. From that level on, we will just use the feature of only considering particles in the appropriate range of h.
  • When recursing, only the h_max of active particles is considered, not all particles.

Notes:

• We will consider c7adb289 separately.
• In a second phase, I'll remove entirely the self + pair tasks and keep only the subs.

Todo:

• Verify RT isn't broken
• Verify MPI runs are happy.
• Port changes to the other hydro schemes.
• Handle particles drifting out of their cells.

Fixes #688 (closed).