Get master building for now

Replaced all calls to fminf and fmaxf by macros

As the Intel compiler can't handle these properly, here is a replacement of all calls to `fmaxf` and `fminf` by a macro that just encapsulates the ternary. This produces assembly call to `vmaxpss` and `vminpss` as expected. 

Any reason not to do this ?

See merge request !235

Documentation of the space and new names for the cells

Some small improvements (hopefully !) to the code documentation and variable descriptions. Implements #208.

See merge request !231

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

cell->super for gravity-related business

I think we need to do this to make sure the gravity tasks are hooked at the right place. Given that the particles and g-particles can be distributed in a different fashion, we should not use the same `super` pointer as tasks might be defined at a different level in the tree. 

What do you think ?

See merge request !226

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