fix: try this density treatment for gravity

[paveltomin]

currently density in gravity term is 0.5(rho_i+rho_j) where i and j are the cells here we are changing it to more commonly used treatment that checks phase presence leading to:

1. 0.5(rho_i+rho_j) when phase is present in both cells
2. 0.5 rho_i when only in i
3. 0.5 rho_j when only in j probably not a big deal but better to be consistent with mainstream way