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

Test 125

Added a new test to help @jwillis with vectorization. 

The old tests are now more thorough as well. 

See merge request !208

Corrected names in testSingle.c to reflect part struct change.



See merge request !209

Time integration mistake ?

Noticed this time integration mistake whilst working on the gravity code. Could this explain the non-conservation of energy ?

See merge request !196

Intrinsic vectorisation

I have implemented vectorised versions of the symmetrical and non-symmetrical density and force interaction functions for the `GADGET_SPH` scheme.

There are also tests for each interaction in `tests/testInteractions.c`.

There are some bug fixes in the `DEFAULT_SPH` along with changes to the `struct part` and minor renaming of variables.

Calculations of the `balsara` switch and `P_over_rho2` are now performed in `hydro_prepare_force()` rather than in `runner_iact_nonsym_force` or `runner_iact_force`.

Note: The !203 `gadget2-part-update` branch should be merged with master prior to this merge request.

See merge request !204

Add entropy to the energy diagnostic file. Use an MPI_Reduce instead of MPI_Allreduce to get the values

Conflicts:
	src/hydro/Default/hydro_iact.h
	src/hydro/Gadget2/hydro.h
	src/hydro/Gadget2/hydro_debug.h
	src/hydro/Gadget2/hydro_iact.h
	src/hydro/Gadget2/hydro_part.h
	tests/testInteractions.c
	tests/testSymInt.c

Read the entropy flag as an array of 6 values

Fix to #192. 

Here is the fix. 

See merge request !207