ENH: surfaceCourantNumber: new finite-area function object

Summary

A new finite-area function object that computes the surface Courant number field at finite-area face centres.

Resolved bugs (If applicable)

N/A

Details of new models (If applicable)

N/A

Risks

N/A Note: the calculation is performed at face centers, not edge centers. For example, liquidFilmFoam's Courant number calculation is performed at edge centres.