Unify story about selection of $\phi$ between Stokes and Navier-Stokes section

[mcastel1]

It is unclear why the definition of $\phi$ in Section "Navier Stokes equations" ($\phi = p^{n-\frac{1}{2}}-p^\star$) is different from that in Section "Stokes equations" ($\phi = p^{n-\frac{1}{2}} - p^* + \frac{\nu}{2}\nabla \cdot \mathbf{u}^\star$)

[jorgensd]

This is because I've adapted the text over time. I should probably unify the story, presenting the version with $\frac{\nu}{2}$ from the start