ART_VISCO : add new scheme for the artifical viscosity + AB_TURB: allow to restart

Add the bossibility to use different type of artificial viscosity
ART_VISCO_CD Cullen & Dehnen
ART_VISCO_MM Morris & Monaghan
ART_VISCO_RO Rossworg

!!! for the moment, the new routine fails in some conditions.
add a new file : art_visc.c

AB_TURB:
restart.c, add instruction to restart when using AB_TURB
allvars.h, move some variables here, in order to be global