Add -P command-line option to overwrite or add program parameters without modifying the YML file, useful for quick tests

Disc patch revived

Implements the changes discussed in #311. 

 - The disc patch example is now 400x400x800 in length with correct internal energy.
 - The accelerations get truncated at |z-z_disc| > 300 with a cosine transition
 - The accelerations are 0 at |z-z_disc| > 380.
 - The calculation is significantly faster thanks to the extraction of constant terms.

See merge request !349

Reset the value of rho_dh to something sensible if no neighbours are found when imposing h=h_max

Fix to #314.

See merge request !351

More soft transition for the truncation of the potential. Set some plotting limits to the solution script.

Do not call the difffloat.py script with the additional deprecated parameters formerly used for rho_dh

Do not call the difffloat.py script with the additional deprecated parameters formerly used for rho_dh

Test periodic bc

This tests the periodic boundary conditions which are generated in `space_getsid()` and used in `runner_dopair1_density`.

An 8x8x8 mesh is generated that is periodic and 26 pair interactions are computed for the following cells:

* All 8 corner cells
* The cell at the centre of each face
* The cell at the centre of each edge

These checks are performed for both unperturbed and perturbed cells.

See merge request !350

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. 

Removed the IDL scripts for the disc patch test case. Updated the python script to use the new range of snapshots.

Added a truncation distance for the disc patch potential bezond which the accelerations tend towards 0. Simplified the code in that potential.

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

Updated the hydrodstatic disc patch example to use a longer slab in z and the correct internal energy.

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