Output the position of the cell in the output of the periodic boundary condition test. Better tolerance requirements.

Corrected mistake in the periodic boundary test that prevented compilation when optimisations are switched off.

Make the difffloat script use a new line in the files to set a limit below which values are ignored as too close to zero.

 - Use the correct value of 'h_max' in the construction of the caches in the vectorized DOPAIR1 routine. (fix #312)
 - Update the test27 unit test to also randomly perturb the smoothing length values.
 - Set a correct value for h_max in the test27 case and not just a loose upper-bound.
 - Call the non-vectorized version of DOPAIR1 for the corner interactions.

See merge request !348

EAGLE_25 now produces the same number of time-steps whether running with or without vectorization. 

Documentation update. The MPI and vectorization macros will be expanded when generating the doxygen files.

Moved the testing into a function and tested each corner cell and each cell at the centre of each face.

Output the particle ID if a difference is found instead of the line in the file and print the particle property that differs.

Created a branching function for DOPAIR1, so that the scalar version is called if it is a corner interaction when the code is vectorised. Also fixed a bug with runner_dopair1_density_vec so that hi_max and hj_max are used to find how many particles to read into the cache.

Disabled GIZMO particle steering for vacuum particles

I disabled particle steering for particles with zero density, as they have weird sound speeds that mess up the steering algorithm. This fixes #271.

See merge request !346