Sanitize smoothing lengths

Here is a proposal to address problems such as #211. 

At the start, after having read in the particles, constructed the top-level cells and all their progenitors we do a quick pass to sanitize the whole thing. If a top-level cell has more than 100000 particles (say) but has an `h_max` that prevents tasks associated with this cell from being split we do the following:

 - Compute the geometric mean and standard deviation of the smoothing lengths in that cell.
 - Limit the smoothing lengths of all particles in this cell to mean+4sigma (say).

Recall that the cells are always split so that the scheduler will be able to push down the tasks at a later point. 
This procedure is applied to the top-level cells only.

Does that make sense to you ?

See merge request !258

Cleanup rho dh and use of the EoS functions

A few more rather unimportant cosmetic changes:

 - moved rho_dh to density.rho_dh everywhere.
 - added a force.f term to contain the transformed value of rho_dh. 
 - When computing the sound speed, I am now using the functions defined in equation_of_state.h everywhere.

The particles are the same size in memory as before despite the change. I had to update the tolerances for the tests as rho_dh changed meaning. 

See merge request !255

Fixed bug in HLLC Riemann solver flux calculation.

This fixes #216 (again, but now all cases).

I basically overlooked a tiny detail in the real GIZMO and changed the order of some operations that cannot be interchanged. Square test is now verified to work with all 3 possible Riemann solvers.

See merge request !257

Updated the ICs for the EAGLE_25 test case

Here are the new ICs if you want to try them out.

See merge request !256

Added conversion of box dimensions

Fixes #217. We need to convert the box size to the internal system of units, not just the particle properties.

See merge request !254

Move rho_dh into the density sub-structure. Update the pressure and sound-speed after kick and drift

Implementation Pressure entropy SPH

Here is a new flavour of SPH that works and gives better result than the default Gadget-2 scheme.

The only change affecting the rest of the code is the addition of the condition in `engine_init_particles()` to create a second density loop over the particles for the special conversions that are required in this scheme. Other than that, it just fits in the rest of the framework.

See merge request !252

Made sure primitive variable velocity is correctly initialized.

This fixes #216. The initial velocities are now initialized (again) in `hydro_convert_quantities` by using `p->v`, before they are used to set the momentum. I also removed the `if` condition from `hydro_end_density`, as we do not longer care if the primitive variables are overwritten after the initialization step.

See merge request !253

(cherry picked from commit e22730e917d6bf96028982d5842f84c517f0835f)

(cherry picked from commit 2c9c86429a9cf195ef671163fd261f9cafb0c910)

(cherry picked from commit ce0055f95a0eb5052081858c883e477bafef2fd2)