Black holes repositioning

This is the last bit of physics of the EAGLE model.

• When looping over the neighbours of a black hole, compute the minimal gravitational potential across all neighbours,
• When drifting, if the minimum is not yourself, move the BH to its new position.
• Force the code to compile with potential calculations switched on when using EAGLE physics.

doc/RTD/source/SubgridModels/EAGLE/index.rst

@@ -577,6 +577,11 @@ Black-hole creation
 Black-hole accretion
 ~~~~~~~~~~~~~~~~~~~~
 
+.. _EAGLE_black_hole_reposition:
+
+Black-hole repositioning
+~~~~~~~~~~~~~~~~~~~~~~~~
+
 .. _EAGLE_black_hole_feedback:
 
 AGN feedback