Set the super-cell to the first cell with interactions.

The super cell is either a non-empty leaf or the first cell for which an interaction task is defined. need to add a condition for gravit tasks once these have been settled.

Separating this from the `mpi_skip` branch as it is not really related and should be tested separately.

See merge request !186

the super cell is either a non-empty leaf or the first cell for which an interaction task is defined. need to add a condition for gravit tasks once these have been settled.

Remove the part<->gpart link checks. These can be un-commented back in when debugging.

As discussed during the meeting, here are some tests that are non-necessary in serious runs.



See merge request !185

Coding style

Sorry Peter here is another big annoying one. 

 - Improved the code quality in `runner_doiact.h` to match the coding standard used elsewhere: declare variable only where they are needed and use constants where possible.
 - Modified many headers to avoid long unnecessary inclusion lists. Should speed up compilation.

Thanks !

See merge request !182

Moved some more hydro parameters as optional

Some more parameters flagged as optional. 

See merge request !184

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