Reverse the loop to find the exit condition in DOPAIR2() to go from the centre to the outside. Follows @jwillis' suggestion in #362

Force compiler to report shadowed variables. Removed all shadowed variables. Added the function scheduler_activate_send() to simplify the unskipping of MPI sends.

Also updated the Pressure-Entropy SPH scheme to use the new definition of the number of neighbours and its derivative. Back to correct results.

Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet for AVX-512 vectors.

Conflicts:
	src/hydro/Gadget2/hydro_iact.h

Partially restored old time step criterion. Isolated flux limiter into a separate file and made it configurable. Removed some superfluous gravity variables.

Moved around the energy gravity update so that it uses the old momentum. Removed a superfluous time step in mflux.

Fixed bug in signal velocity computation. Added relative velocity of fluid w.r.t. particle velocity to timestep criterion.

Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet for AVX-512 vectors.

Conflicts:
	src/hydro/Gadget2/hydro_iact.h

Use intrinsics for vector operations as arithmetic overloading is not supported by the Intel compiler yet for AVX-512 vectors.

Updated the way we converge towards the smoothing length. We now use a Newton-Raphson scheme. This applies to the Gadget-2 scheme.