Correct the equation for the entropy time derivative in GADGET2_SPH

This fixes #183 meaning that energy is now correctly conserved.

It also:
 - Adds a parameter file and run script for the perturbed box test case.
 - Adds entropy to the diagnostics file.
 - Replace the MPI_AllReduce in the statistics collection by an MPI_Reduce since only rank 0 writes anyway. 
 - Gives a better documentation for the `approx_exp()` function.
 - Adds an accuracy test for the approximate maths functions (currently only exp())
 - Adds a test no make sure the symmetric and non-symmetric versions of the SPH interaction routines give the same answer. That was the origin of the energy non-conservation bug.

See merge request !210