Correctly update the counters when adding a spart to the global array. Handle the case where we run out of free slots.

Correctly link the newly created star to the ddestroyed gpart. Can survive time integration for a few steps. Crashes at next rebuild still.

Version can now create multiple particles and put them in the correct cell array. Code can survive multiple rebuilds.

This reverts commit e44d0632.

C++ build fix: seems that the file c++config.h undef's the max and min macros, the solution is to include something that pulls in that file before we do our own defines (in minmax.h)

See: https://gcc.gnu.org/ml/libstdc++/2016-01/msg00025.html and #489

Change the task splitting criterion for gravity tasks

Closes #457

See merge request !667