Modify averaged equations by adding pressure correction

This modification implements the following equation

$$\nabla \cdot \boldsymbol{\bar{u}} = 0 \quad \text{and} \quad - \mu \Delta \boldsymbol{\bar{u}} + \phi \nabla \bar{p} = \mathbf{f}_\text{drag}$$

which can be reformulated as

$$\nabla \cdot \boldsymbol{\bar{u}} = 0 \quad \text{and} \quad - \mu \Delta \boldsymbol{\bar{u}} + \nabla \bar{p} = \mathbf{f}_\text{drag} + (1-\phi) \nabla \bar{p}$$

Therefore, the additional term `

$(1-\phi) \nabla \bar{p}$

is added to the momentum source term.