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

Gadget2 part update

A compilation of changes from @jwillis and myself re-factoring a bit the particle definition in the Gadget version. 
This has knock-on consequences on the other schemes.

Also, a initial vectorization of the Gadget-2 interaction routines will be pushed in with the associated tests. These will be refined and actually used in a separate branch.

See merge request !203

Renaming of variables in cell and space struct

Renamed 'h' in cell struct to 'width' and renamed 'h' and 'ih' in the space struct to 'width' and 'iwidth'.



See merge request !206

Isothermal potential

added some more external potentials to gravity, plus a test for energy and angular momentum conservation in the IsothermalPotential/GravityOnly example.

See merge request !205

Renamed 'h' in cell struct to 'width' and renamed 'h' and 'ih' in the space struct to 'width' and 'iwidth'.

Created function pointers to test all different types of interactions: density (non-sym/sym); force (non-sym/sym) for serial and vectorised versions.

First attempt at a vectorised version of the symmetrical force interaction. Fixed signal velocity assignment in non-sym version.