ENH: FO: Lamb vector and its divergence

If applied: This commit allows the user to compute: - the Lamb vector (https://en.wikipedia.org/wiki/Lamb_vector), - (optionally) its divergence - on-the-fly or via postProcess utility - for a given volVectorField (one per functionObject entry)

• Why: The motivation is the literature-reported quantitative connection between the Lamb vector (divergence) and the spatially localised instantaneous fluid motions, e.g. high- and low-momentum fluid parcels, which possess considerable level of capacity to affect the rate of change of momentum, and to generate forces such as drag.

• Verification:

  • Smooth-wall plane channel flow case (Moser et al. 1999) by
    • Curtis et al. (2008) On the Lamb vector divergence in Navier–Stokes flows, doi:10.1017/S0022112008002760

• What's next:

  • The verification-show case
  • Extended code guide entry titled "Lamb vector"

• Related:

  • "fvc::curl(U)^U" is computed twice when "divergence_" is on. It will be reduced one.