Typo in sub_subset that leads to a crash in the isothermal sphere case.

This bug has been there forever. @jregan uncovered it today by running the isothermal sphere case which has a rather deep tree. 

Re-assign to me if happy.

See merge request !183

Task sub split

This implements #175 
While at it I also removed some dead code (including #174) and cleaned up the linking of libraries in the Makefile. For some reasons we had still blas in lapack in there.

Finally I have also removed some commented-out old gravity stuff. That should make the diff for the gravity branch much easier to read when it becomes time to merge that in.

See merge request !180

Unified the atomic calls everywhere using the defined macros

Some more code clean-up. I have modified all the calls to __sync_* to use the defined atomic macros. Makes the code easier to read. 

Note that in the case of the timer function, moving to the defined macros changed the return value (++i instead of i++). But we don't use the return value anyway. 

Re-assign to me if you are happy to have this merged in.

See merge request !181

Queue fix

Separate adding and getting a task. Uses a double-ended queue to buffer incoming tasks. The buffer is then added to the actual queue whenever queue_gettask is called.

Peter, can you check if this doesn't break anything? Thanks!

See merge request !178