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