Fixed density, acceleration, energy_RHS, and artificial viscosity functions.  Moved density, acceleration, and energy_RHS functions to sph_class.cpp.  Artificial Viscosity function now computes pairwise viscosity.  Currently it is called each time it is needed, but this is only slower by a factor of 2 (although since AV is pairwise-antisymmetric, the matrix method could be sped up by a factor of 2).  Also added attributes for density, pressure, cs, acceleration, etc, in the particle classes.