Set engine_max_parts_per_ghost from the parameter file

Allow engine_max_parts_per_ghost to be changed with the .yml, to fix a crash with >10^7 particle planetary simulations.

doc/RTD/source/ParameterFiles/parameter_description.rst

@@ -656,6 +656,17 @@ which stops these from being done at the scale of the leaf cells, of which
 there can be a large number. In this case cells with gravity tasks must be at
 least 4 levels above the leaf cells (when possible).
 
+To control the depth at which the ghost tasks are placed, there are
+two parameters (one for the gas, one for the stars). These specify the
+maximum number of particles allowed in such a task before splitting
+into finer ones. These parameters are:
+
+.. code:: YAML
+
+  engine_max_parts_per_ghost:   1000
+  engine_max_sparts_per_ghost:  1000
+
+
 Extra space is required when particles are created in the system (to the time
 of the next rebuild). These are controlled by: