Improved Superficial velocity calculation with paraview

[malihass]

Use paraview module to extract slice at midpoint in the liquid phase and average gas velocity over the slice
A volume flow rate through the slice is computed and divided by the surface area of the slice.
The creation of the slice is best done with paraview which requires modifying a little bit how the installation is done.

This is how the superficial velocity is traditionally computed. The previous implementation was approximating this operation.

I ran tests with the bubble column tutorial at times 0.5, 0.6, 0.7, 0.8
The old method gives [0.002272, 0.001821, 0.002458, 0.002894] and takes 0.32s to run on a Kestrel CPU.
The new one gives [0.002271, 0.00182, 0.002455, 0.002888] and takes 0.88s to run on a Kestrel CPU.

The default behavior is still to use the old method and a parameter use_pv allows to use paraview.