Promoted the external gravity tasks to a self sub-type such that the super pointer can be set correctly.

Re-factored the whole external potential mechanism to match what is done for cooling. Renamed 'disk' to 'disc'.

Collect the radiated energy of all particles when computing statistics and output the result in the log file.

Replaced all calls to fminf and fmaxf by the macros that was added for the integer min/max functions.

Broke one of my own rules by doing too much in a single commit: implemented runner_extra_ghost, moved gradient calculations for both gradient types to new gradient loop, fixed bug in engine.c involving the number of links (which is larger with an extra loop), streamlined gradient methods, documented most GIZMO specific files. Second loop works.

Now at least as fast as tkask version