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.