Added a command-line option '-d' to execute a dry run which read the parameter file and opens the ICs to check whether everything is OK.

Parallel gpart sort

That's essentially a copy-paste of the part version. 

Indices are still of type `size_t` to be compatible with the rest of the code. We can move to `int` later on.

See merge request !128

More physical definition of SPH kernel functions

This "solves" #97. 

 - I have split the SPH and gravity kernels into two separate files for ease of use. That explains a lot of the file changes below.
 - Corrected a small typo in the hydro.h files. The mistake was not leading to wrong physics results but would slow down convergence towards the correct h in the ghost task.
 - When switching kernel function, the resolution is not modified any more. This was the main point of #97.
 - The documentation of the kernels has increased and I have updated the .tex documents and figures to be completely consistent with the code.

In normal mode (i.e. without editing const.h) the results are identical. 

See merge request !138

Added functionality to throw errors when incorrect indentation used inside a section and when a standalone parameter is indented.