PU: update theory

The viscosity in the theory does not correspond to the implementation.

Here is the new version (before the mass before the viscosity was not present and the parenthesis was closing after it)

theory/SPH/Flavours/sph_flavours.tex

@@ -462,8 +462,8 @@ motion:
   \frac{\mathrm{d} \mathbf{v}_i}{\mathrm{d} t} = -\sum_j (\gamma - 1)^2 m_j u_j
 u_i
 	 &\left[\frac{f_{ij}}{\bar{P}_i} \nabla_i W_{ij}(h_i) ~+ \right. \nonumber \\
-	       &\frac{f_{ji}}{\bar{P}_j} \nabla_i W_{ji}(h_j) ~+ \nonumber \\
-	       & \left.\nu_{ij}\bar{\nabla_i W_{ij}}\right]~.
+	       &\left.\frac{f_{ji}}{\bar{P}_j} \nabla_i W_{ji}(h_j)\right] ~+ \nonumber \\
+	       & m_j \nu_{ij}\bar{\nabla_i W_{ij}}~.
   \label{eq:sph:pu:eom}
 \end{align}
 which includes the Monaghan artificial viscosity term and Balsara switch in