GCC 5x doesn't like this now we have -Wextra, if offends make count size_t as well

Only drift the particles that need drifting

Here is an algorithmic speed-up. The plan is to drift only the cells that contain an active particles and the cells that have at least one neighbour with an active particle. 

 - If this condition is not met then `runner_do_drift()` just returns without performing any operation. 

 - If the condition is met, `runner_do_drift()` moves all particles (as before) from the last point in time where a drift was performed. 

We record for each cell the last time a drift was done such that we now by how much to drift things when needed.

Two exceptions to this rule: before reconstructing and before duping a snapshot,  we drift all particles as we need correct position information at these two stages.

Note that I have made this an engine policy to ease the testing. Drifting all particles can be restored by setting the right flag.

@nnrw56  could you check that the function `cell_is_drift_needed()` makes sense and fully does what it says it does ? Also could you let me know whether you also agree with the whole logic and implementation.


See merge request !225

Avoid recursion in the tasks if no particle is active.

Does exactly what it says. We don't want to go through all the cells recursively just to find the handful that need this task to act upon. 

Big improvements when added to the `kill_drift` branch on the EAGLE_25 case. 

See merge request !230

Make the entry affinity pointer a static variable in a function rather a global

Here is what I hinted to in #204. I find this clearer to read. Do you agree with that change ?

See merge request !229

make update of s->maxdepth atomic.

Fixes #205.

See merge request !227

Removed the (commented-out) branch-free versions of the kernel functions. Also improved documentation.

Added equation of state check to Riemann solver.

The currently implemented Riemann solvers only work with an ideal gas equation of state. I have added a compile time check to riemann.h that throws an error if the selected equation of state is not EOS_IDEAL_GAS. This way, it is impossible to compile GIZMO_SPH with a wrong equation of state.

See merge request !224

Merge Gizmo-SPH into the master branch



See merge request !223