Merge branch 'energy_conservation' into 'master'
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