Added some more comments to 3D algorithm, and fixed a bug that affected some of the degenerate cases.

Replaced global variables by a new hydro_space struct that is part of space. This struct is then passed on to hydro_init_part, and used to initialize the Voronoi cells. For non SHADOWFAX_SPH schemes, this struct is empty. Replaced random_uniform declarations in some unit tests with the tools.h version. Added some more comments to 3D Voronoi code; it is now almost completely documented (meaning I almost completely understand it now).

Only repartition when required

Only repartition when the previous step processed some large fraction of all the
particles, and then only when the loads between the ranks are out of balance.
This is for several reasons:

  * Repartitioning is expensive, so should only be done when necessary.
  * Frequent repartitioning with multi-dt is not necessary (for the EAGLE volumes anyway).
  * It is more representative to check the load balance when all tasks have been ran.

The load balance is determined from the user CPU time per step (including
the CPU time from all threads). We exclude the system time as that is not
down to processing and tends to even out the ranks artificially, much as
elapsed time does (since we wait for all the MPI tasks to come together).

The load imbalance allowed is determined by the parameter `DomainDecomposition:trigger`,
this can also be set to a number greater than one, in which case the old
repartitioning scheme of every 'trigger' steps will be used (previously trigger
was always 100).


See merge request !290

Added a lot of comments to 3D Voronoi algorithm, fixed bug in testVoronoi3D. Disabled expensive checks. Some basic 3D moving mesh tests now work.

Set the unnecessary dimensions of the box in 1D or 2D to safe values to prevent unnecessary crashes due to lack of cells along this particular axis.

Removed duplicated entries in gitignore file. Updated ICs for 2D Sod shock (See #278). Reduced max_dt for 1D sedov explosion

Added pressure to entropy and pressure to internal energy functions to the equation of state definition.

Include @bvandenbroucke's typo corrections in dimension.h as well as the sphere to radius calculation functions.

Added actual grid test to testVoronoi3D; degenerate case crashes. Started documenting 3D Voronoi algorithm to refresh my memory about how it works.

cputimes output file to avoid an MPI call each step. General tidyups
to remove unneccesary values and to keep APIs clean.

Fixed wrong box size in 2D Sod shock that caused SHADOWFAX_SPH to crash. Added flag in const.h to enable or disable cell motion steering. Made sure cell centroids have all their coordinates initialized.

Fixed bug in 2D Voronoi algorithm. Sod shock now works (with fixed cells). Moved gradient and slope limiter defines for SHADOWFAX_SPH to const.h.

Fixed small mistake in 2D Voronoi neighbour calculation. Added check for neighbour relations to unit test.

Realized I forgot to implement neighbour relations in 2D Voronoi algorithm. Implemented them. 2D Sod shock still doesn't work.

Fixed a bug in 2D Voronoi algorithm, added a lot of debugging output possibilities. 2D algorithm now works for 100 uniform random generators in a periodic box.

Finished first part of 2D Voronoi intersection algorithm. Now *only* need to write the second part. And maybe deal with degeneracies...

Stop repartitioning in the step after a repartition, that makes no sense and is unlikely to give interesting CPU times. Also remove some development output from timestep file

Conflicts:
	examples/parameter_example.yml
	src/engine.c

The repartitioning is now delayed to the next step. Need to check
that %2 is still appropriate.

CPU time fraction. Also allow the balance test to be done if fewer
than the complete number of particles interact. Finally capture the
CPU times to a file so that the balance can be assessed. This is
only done when task timings are also enabled.

3D SHADOWFAX_SPH compiles again (but does not really work). Re-introduced global variables to store the box size.