First iteration of the "improved SPH" implementation. The code now uses a...

First iteration of the "improved SPH" implementation. The code now uses a (Rosswog et al, 2000) viscosity term with a (Balsara , 1995) switch and the parameters given by (Price, 2004). 
Thermal conductivity is implemented following (Price, 2008) without the switch.
Both scalar and vectorized versions have been implemented. 
The code now gives a much better answer on the Sod shock. More testing will be required on more advanced tests to tune the parameters.

When "LEGACY_GADGET2_SPH" is defined, the code defaults to the old gadget-2 version of SPH.




Former-commit-id: 840aa8ecfea0a60816f64b8e5818162f2f9006d2